fix: Fix Spanner DatabaseSessionService support

Introduce `DynamicPickleType` to handle session actions, using sqlalchemy-spanner `SpannerPickleType` when the database dialect is Spanner.

Connects to a Spanner database to store session data persistently in tables.
# Example using Spanner database:
`session_service = DatabaseSessionService(db_url="spanner+spanner:///projects/project-id/instances/instance-id/databases/database-id")`

# Example adk web command:
`adk web --session_service_uri="spanner+spanner:///projects/project-id/instances/instance-id/databases/database-id"`

PiperOrigin-RevId: 797416610

pyproject.toml

@@ -48,6 +48,7 @@ dependencies = [
   "python-dateutil>=2.9.0.post0, <3.0.0",           # For Vertext AI Session Service
   "python-dotenv>=1.0.0, <2.0.0",                   # To manage environment variables
   "requests>=2.32.4, <3.0.0",
+  "sqlalchemy-spanner>=1.14.0",                     # Spanner database session service
   "sqlalchemy>=2.0, <3.0.0",                        # SQL database ORM
   "starlette>=0.46.2, <1.0.0",                      # For FastAPI CLI
   "tenacity>=8.0.0, <9.0.0",                        # For Retry management

src/google/adk/sessions/database_session_service.py

@@ -18,6 +18,7 @@ from datetime import datetime
 from datetime import timezone
 import json
 import logging
+import pickle
 from typing import Any
 from typing import Optional
 import uuid
@@ -104,6 +105,33 @@ class PreciseTimestamp(TypeDecorator):
     return self.impl
 
 
+class DynamicPickleType(TypeDecorator):
+  """Represents a type that can be pickled."""
+
+  impl = PickleType
+
+  def load_dialect_impl(self, dialect):
+    if dialect.name == "spanner+spanner":
+      from google.cloud.sqlalchemy_spanner.sqlalchemy_spanner import SpannerPickleType
+
+      return dialect.type_descriptor(SpannerPickleType)
+    return self.impl
+
+  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":
+        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":
+        return pickle.loads(value)
+    return value
+
+
 class Base(DeclarativeBase):
   """Base class for database tables."""
 
@@ -209,7 +237,7 @@ class StorageEvent(Base):
       PreciseTimestamp, default=func.now()
   )
   content: Mapped[dict[str, Any]] = mapped_column(DynamicJSON, nullable=True)
-  actions: Mapped[MutableDict[str, Any]] = mapped_column(PickleType)
+  actions: Mapped[MutableDict[str, Any]] = mapped_column(DynamicPickleType)
 
   long_running_tool_ids_json: Mapped[Optional[str]] = mapped_column(
       Text, nullable=True