feat: migrate invocation_context to callback_context

Update plugin manager and built-in plugins to prioritize CallbackContext. Keep InvocationContext access for legacy plugins with adapter. Change callback docs/tests to cover the new context.

PiperOrigin-RevId: 818822267

tests/unittests/plugins/test_base_plugin.py

@@ -103,15 +103,19 @@ async def test_base_plugin_default_callbacks_return_none():
   assert (
       await plugin.on_user_message_callback(
           user_message=mock_user_message,
-          callback_context=mock_context,
+          invocation_context=mock_context,
       )
       is None
   )
-  assert await plugin.before_run_callback(callback_context=mock_context) is None
-  assert await plugin.after_run_callback(callback_context=mock_context) is None
+  assert (
+      await plugin.before_run_callback(invocation_context=mock_context) is None
+  )
+  assert (
+      await plugin.after_run_callback(invocation_context=mock_context) is None
+  )
   assert (
       await plugin.on_event_callback(
-          callback_context=mock_context, event=mock_context
+          invocation_context=mock_context, event=mock_context
       )
       is None
   )
@@ -196,21 +200,25 @@ async def test_base_plugin_all_callbacks_can_be_overridden():
   assert (
       await plugin.on_user_message_callback(
           user_message=mock_user_message,
-          callback_context=mock_callback_context,
+          invocation_context=mock_invocation_context,
       )
       == "overridden_on_user_message"
   )
   assert (
-      await plugin.before_run_callback(callback_context=mock_callback_context)
+      await plugin.before_run_callback(
+          invocation_context=mock_invocation_context
+      )
       == "overridden_before_run"
   )
   assert (
-      await plugin.after_run_callback(callback_context=mock_callback_context)
+      await plugin.after_run_callback(
+          invocation_context=mock_invocation_context
+      )
       == "overridden_after_run"
   )
   assert (
       await plugin.on_event_callback(
-          callback_context=mock_callback_context, event=mock_event
+          invocation_context=mock_invocation_context, event=mock_event
       )
       == "overridden_on_event"
   )

tests/unittests/plugins/test_global_instruction_plugin.py

@@ -43,7 +43,7 @@ async def test_global_instruction_plugin_with_string():
   mock_invocation_context.session = mock_session
 
   mock_callback_context = Mock(spec=CallbackContext)
-  mock_callback_context._invocation_context = mock_invocation_context
+  mock_callback_context.invocation_context = mock_invocation_context
 
   llm_request = LlmRequest(
       model="gemini-1.5-flash",
@@ -70,7 +70,7 @@ async def test_global_instruction_plugin_with_instruction_provider():
   """Test GlobalInstructionPlugin with an InstructionProvider function."""
 
   async def build_global_instruction(readonly_context: ReadonlyContext) -> str:
-    return f"You are assistant for user {readonly_context.user_id}."
+    return f"You are assistant for user {readonly_context.session.user_id}."
 
   plugin = GlobalInstructionPlugin(global_instruction=build_global_instruction)
 
@@ -83,8 +83,7 @@ async def test_global_instruction_plugin_with_instruction_provider():
   mock_invocation_context.session = mock_session
 
   mock_callback_context = Mock(spec=CallbackContext)
-  mock_callback_context._invocation_context = mock_invocation_context
-  mock_callback_context.user_id = "alice"
+  mock_callback_context.invocation_context = mock_invocation_context
 
   llm_request = LlmRequest(
       model="gemini-1.5-flash",
@@ -120,7 +119,7 @@ async def test_global_instruction_plugin_empty_instruction():
   mock_invocation_context.session = mock_session
 
   mock_callback_context = Mock(spec=CallbackContext)
-  mock_callback_context._invocation_context = mock_invocation_context
+  mock_callback_context.invocation_context = mock_invocation_context
 
   llm_request = LlmRequest(
       model="gemini-1.5-flash",
@@ -157,7 +156,7 @@ async def test_global_instruction_plugin_leads_existing():
   mock_invocation_context.session = mock_session
 
   mock_callback_context = Mock(spec=CallbackContext)
-  mock_callback_context._invocation_context = mock_invocation_context
+  mock_callback_context.invocation_context = mock_invocation_context
 
   llm_request = LlmRequest(
       model="gemini-1.5-flash",
@@ -192,7 +191,7 @@ async def test_global_instruction_plugin_prepends_to_list():
   mock_invocation_context.session = mock_session
 
   mock_callback_context = Mock(spec=CallbackContext)
-  mock_callback_context._invocation_context = mock_invocation_context
+  mock_callback_context.invocation_context = mock_invocation_context
 
   llm_request = LlmRequest(
       model="gemini-1.5-flash",