feat: Integrating Plugin with ADK

This change integrates the plugin system with ADK. PluginManager is attached to the invocation context similar to session/artifact/memory.

It includes integrations with following ADK internal callbacks:

* App callbacks: Integrated in the BaseRunner class, in run_async and run_live
* On Message callbacks: Integrated in the BaseRunner class, triggers on run_async.
* Agent callbacks: Integrated in the BaseAgent class. Leveraging the existing *callback functions
* Model callbacks: Integrating in the base_llm_flow.
* Tool callbacks: Integrated in functions.py, wrapped around the code for agent tool_callbacks

Sample code to use plugins:

```python
# Add plugins to Runner

runner = Runner(
      app_name="my-app",
      agent=root_agent,
      artifact_service=artifact_service,
      session_service=session_service,
      memory_service=memory_service,
      plugins=[
        MySamplePlugin(),
        LoggingPlugin(),
      ],
  )

```

PiperOrigin-RevId: 781746586

tests/unittests/flows/llm_flows/test_plugin_model_callbacks.py

@@ -0,0 +1,128 @@
+# 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 typing import Optional
+
+from google.adk.agents import Agent
+from google.adk.agents.callback_context import CallbackContext
+from google.adk.models import LlmRequest
+from google.adk.models import LlmResponse
+from google.adk.plugins.base_plugin import BasePlugin
+from google.genai import types
+import pytest
+
+from ... import testing_utils
+
+
+class MockPlugin(BasePlugin):
+  before_model_text = 'before_model_text from MockPlugin'
+  after_model_text = 'after_model_text from MockPlugin'
+
+  def __init__(self, name='mock_plugin'):
+    self.name = name
+    self.enable_before_model_callback = False
+    self.enable_after_model_callback = False
+    self.before_model_response = LlmResponse(
+        content=testing_utils.ModelContent(
+            [types.Part.from_text(text=self.before_model_text)]
+        )
+    )
+    self.after_model_response = LlmResponse(
+        content=testing_utils.ModelContent(
+            [types.Part.from_text(text=self.after_model_text)]
+        )
+    )
+
+  async def before_model_callback(
+      self, *, callback_context: CallbackContext, llm_request: LlmRequest
+  ) -> Optional[LlmResponse]:
+    if not self.enable_before_model_callback:
+      return None
+    return self.before_model_response
+
+  async def after_model_callback(
+      self, *, callback_context: CallbackContext, llm_response: LlmResponse
+  ) -> Optional[LlmResponse]:
+    if not self.enable_after_model_callback:
+      return None
+    return self.after_model_response
+
+
+CANONICAL_MODEL_CALLBACK_CONTENT = 'canonical_model_callback_content'
+
+
+def canonical_agent_model_callback(**kwargs) -> Optional[LlmResponse]:
+  return LlmResponse(
+      content=testing_utils.ModelContent(
+          [types.Part.from_text(text=CANONICAL_MODEL_CALLBACK_CONTENT)]
+      )
+  )
+
+
+@pytest.fixture
+def mock_plugin():
+  return MockPlugin()
+
+
+def test_before_model_callback_with_plugin(mock_plugin):
+  """Tests that the model response is overridden by before_model_callback from the plugin."""
+  responses = ['model_response']
+  mock_model = testing_utils.MockModel.create(responses=responses)
+  mock_plugin.enable_before_model_callback = True
+  agent = Agent(
+      name='root_agent',
+      model=mock_model,
+  )
+
+  runner = testing_utils.InMemoryRunner(agent, plugins=[mock_plugin])
+  assert testing_utils.simplify_events(runner.run('test')) == [
+      ('root_agent', mock_plugin.before_model_text),
+  ]
+
+
+def test_before_model_fallback_canonical_callback(mock_plugin):
+  """Tests that when plugin returns empty response, the model response is overridden by the canonical agent model callback."""
+  responses = ['model_response']
+  mock_plugin.enable_before_model_callback = False
+  mock_model = testing_utils.MockModel.create(responses=responses)
+  agent = Agent(
+      name='root_agent',
+      model=mock_model,
+      before_model_callback=canonical_agent_model_callback,
+  )
+
+  runner = testing_utils.InMemoryRunner(agent)
+  assert testing_utils.simplify_events(runner.run('test')) == [
+      ('root_agent', CANONICAL_MODEL_CALLBACK_CONTENT),
+  ]
+
+
+def test_before_model_callback_fallback_model(mock_plugin):
+  """Tests that the model response is executed normally when both plugin and canonical agent model callback return empty response."""
+  responses = ['model_response']
+  mock_plugin.enable_before_model_callback = False
+  mock_model = testing_utils.MockModel.create(responses=responses)
+  agent = Agent(
+      name='root_agent',
+      model=mock_model,
+  )
+
+  runner = testing_utils.InMemoryRunner(agent, plugins=[mock_plugin])
+  assert testing_utils.simplify_events(runner.run('test')) == [
+      ('root_agent', 'model_response'),
+  ]
+
+
+if __name__ == '__main__':
+  pytest.main([__file__])