fix: Update DynamicPickleType to support MySQL dialect

Merge https://github.com/google/adk-python/pull/3282

The `process_bind_param` and `process_result_value` methods in the `DynamicPickleType` class have been modified to handle MySQL dialect in addition to Spanner. This change ensures that pickled values are correctly processed for both database types.

**Please ensure you have read the [contribution guide](https://github.com/google/adk-python/blob/main/CONTRIBUTING.md) before creating a pull request.**

### Link to Issue or Description of Change

**1. Link to an existing issue (if applicable):**

- Closes: #3283

**2. Or, if no issue exists, describe the change:**

_If applicable, please follow the issue templates to provide as much detail as
possible._

**Problem:**
When using `DatabaseSessionService` with MySQL backend in google-adk v1.17.0, the application crashes with the following error: app.resources.runner:event_generator:260 - Error in event_generator: (builtins.TypeError) 'tuple' object cannot be interpreted as an integer
<img width="1237" height="129" alt="image" src="https://github.com/user-attachments/assets/0a5fc223-600a-4a92-8443-4d37fb1267f6" />

Root cause: The `DynamicPickleType` class in `database_session_service.py` configures MySQL dialect to use `LONGBLOB` for storing pickled data (line 117-118), but the `process_bind_param` and `process_result_value` methods only handle pickle serialization/deserialization for Spanner dialect, not MySQL. This causes MySQL to attempt storing raw Python objects instead of pickled bytes, leading to serialization errors and potential data corruption.

**Solution:**
Added MySQL to the pickle serialization logic in both `process_bind_param` and `process_result_value` methods, treating it the same way as Spanner dialect. This ensures that:
- Data is properly pickled to bytes before being stored in MySQL's LONGBLOB column
- Data is properly unpickled when retrieved from the database
- No breaking changes to existing functionality for other dialects (SQLite, PostgreSQL)

### Testing Plan

_Please describe the tests that you ran to verify your changes. This is required
for all PRs that are not small documentation or typo fixes._

**Unit Tests:**

- [x] I have added or updated unit tests for my change.
- [x] All unit tests pass locally.

**Summary of `pytest` results:**
<img width="929" height="306" alt="image" src="https://github.com/user-attachments/assets/3d548b96-ac49-4101-8405-a289a722293c" />

**Manual End-to-End (E2E) Tests:**

_Please provide instructions on how to manually test your changes, including any
necessary setup or configuration. Please provide logs or screenshots to help
reviewers better understand the fix._

### Checklist

- [x]  I have read the [CONTRIBUTING.md](https://github.com/google/adk-python/blob/main/CONTRIBUTING.md) document.
- [x]  I have performed a self-review of my own code.
- [x]  I have commented my code, particularly in hard-to-understand areas.
- [x]  I have added tests that prove my fix is effective or that my feature works.
- [x]  New and existing unit tests pass locally with my changes.
- [x]  I have manually tested my changes end-to-end.
- [x]  Any dependent changes have been merged and published in downstream modules.

### Additional context

_Add any other context or screenshots about the feature request here._

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/3282 from hung12ct:fix/mysql-pickle-serialization d9df37adb7dfbbfd8502a0fe65c4f8bca3d0d978
PiperOrigin-RevId: 825834360

src/google/adk/sessions/database_session_service.py

@@ -125,14 +125,14 @@ class DynamicPickleType(TypeDecorator):
   def process_bind_param(self, value, dialect):
     """Ensures the pickled value is a bytes object before passing it to the database dialect."""
     if value is not None:
-      if dialect.name == "spanner+spanner":
+      if dialect.name in ("spanner+spanner", "mysql"):
         return pickle.dumps(value)
     return value
 
   def process_result_value(self, value, dialect):
     """Ensures the raw bytes from the database are unpickled back into a Python object."""
     if value is not None:
-      if dialect.name == "spanner+spanner":
+      if dialect.name in ("spanner+spanner", "mysql"):
         return pickle.loads(value)
     return value
 

tests/unittests/sessions/test_dynamic_pickle_type.py

@@ -0,0 +1,181 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from __future__ import annotations
+
+import pickle
+from unittest import mock
+
+from google.adk.sessions.database_session_service import DynamicPickleType
+import pytest
+from sqlalchemy import create_engine
+from sqlalchemy.dialects import mysql
+
+
+@pytest.fixture
+def pickle_type():
+  """Fixture for DynamicPickleType instance."""
+  return DynamicPickleType()
+
+
+def test_load_dialect_impl_mysql(pickle_type):
+  """Test that MySQL dialect uses LONGBLOB."""
+  # Mock the MySQL dialect
+  mock_dialect = mock.Mock()
+  mock_dialect.name = "mysql"
+
+  # Mock the return value of type_descriptor
+  mock_longblob_type = mock.Mock()
+  mock_dialect.type_descriptor.return_value = mock_longblob_type
+
+  impl = pickle_type.load_dialect_impl(mock_dialect)
+
+  # Verify type_descriptor was called once with mysql.LONGBLOB
+  mock_dialect.type_descriptor.assert_called_once_with(mysql.LONGBLOB)
+  # Verify the return value is what we expect
+  assert impl == mock_longblob_type
+
+
+def test_load_dialect_impl_spanner(pickle_type):
+  """Test that Spanner dialect uses SpannerPickleType."""
+  # Mock the spanner dialect
+  mock_dialect = mock.Mock()
+  mock_dialect.name = "spanner+spanner"
+
+  with mock.patch(
+      "google.cloud.sqlalchemy_spanner.sqlalchemy_spanner.SpannerPickleType"
+  ) as mock_spanner_type:
+    pickle_type.load_dialect_impl(mock_dialect)
+    mock_dialect.type_descriptor.assert_called_once_with(mock_spanner_type)
+
+
+def test_load_dialect_impl_default(pickle_type):
+  """Test that other dialects use default PickleType."""
+  engine = create_engine("sqlite:///:memory:")
+  dialect = engine.dialect
+  impl = pickle_type.load_dialect_impl(dialect)
+  # Should return the default impl (PickleType)
+  assert impl == pickle_type.impl
+
+
+@pytest.mark.parametrize(
+    "dialect_name",
+    [
+        pytest.param("mysql", id="mysql"),
+        pytest.param("spanner+spanner", id="spanner"),
+    ],
+)
+def test_process_bind_param_pickle_dialects(pickle_type, dialect_name):
+  """Test that MySQL and Spanner dialects pickle the value."""
+  mock_dialect = mock.Mock()
+  mock_dialect.name = dialect_name
+
+  test_data = {"key": "value", "nested": [1, 2, 3]}
+  result = pickle_type.process_bind_param(test_data, mock_dialect)
+
+  # Should be pickled bytes
+  assert isinstance(result, bytes)
+  # Should be able to unpickle back to original
+  assert pickle.loads(result) == test_data
+
+
+def test_process_bind_param_default(pickle_type):
+  """Test that other dialects return value as-is."""
+  mock_dialect = mock.Mock()
+  mock_dialect.name = "sqlite"
+
+  test_data = {"key": "value"}
+  result = pickle_type.process_bind_param(test_data, mock_dialect)
+
+  # Should return value unchanged (SQLAlchemy's PickleType handles it)
+  assert result == test_data
+
+
+def test_process_bind_param_none(pickle_type):
+  """Test that None values are handled correctly."""
+  mock_dialect = mock.Mock()
+  mock_dialect.name = "mysql"
+
+  result = pickle_type.process_bind_param(None, mock_dialect)
+  assert result is None
+
+
+@pytest.mark.parametrize(
+    "dialect_name",
+    [
+        pytest.param("mysql", id="mysql"),
+        pytest.param("spanner+spanner", id="spanner"),
+    ],
+)
+def test_process_result_value_pickle_dialects(pickle_type, dialect_name):
+  """Test that MySQL and Spanner dialects unpickle the value."""
+  mock_dialect = mock.Mock()
+  mock_dialect.name = dialect_name
+
+  test_data = {"key": "value", "nested": [1, 2, 3]}
+  pickled_data = pickle.dumps(test_data)
+
+  result = pickle_type.process_result_value(pickled_data, mock_dialect)
+
+  # Should be unpickled back to original
+  assert result == test_data
+
+
+def test_process_result_value_default(pickle_type):
+  """Test that other dialects return value as-is."""
+  mock_dialect = mock.Mock()
+  mock_dialect.name = "sqlite"
+
+  test_data = {"key": "value"}
+  result = pickle_type.process_result_value(test_data, mock_dialect)
+
+  # Should return value unchanged (SQLAlchemy's PickleType handles it)
+  assert result == test_data
+
+
+def test_process_result_value_none(pickle_type):
+  """Test that None values are handled correctly."""
+  mock_dialect = mock.Mock()
+  mock_dialect.name = "mysql"
+
+  result = pickle_type.process_result_value(None, mock_dialect)
+  assert result is None
+
+
+@pytest.mark.parametrize(
+    "dialect_name",
+    [
+        pytest.param("mysql", id="mysql"),
+        pytest.param("spanner+spanner", id="spanner"),
+    ],
+)
+def test_roundtrip_pickle_dialects(pickle_type, dialect_name):
+  """Test full roundtrip for MySQL and Spanner: bind -> result."""
+  mock_dialect = mock.Mock()
+  mock_dialect.name = dialect_name
+
+  original_data = {
+      "string": "test",
+      "number": 42,
+      "list": [1, 2, 3],
+      "nested": {"a": 1, "b": 2},
+  }
+
+  # Simulate bind (Python -> DB)
+  bound_value = pickle_type.process_bind_param(original_data, mock_dialect)
+  assert isinstance(bound_value, bytes)
+
+  # Simulate result (DB -> Python)
+  result_value = pickle_type.process_result_value(bound_value, mock_dialect)
+  assert result_value == original_data