feat: Allow custom part converters in A2A classes

This change introduces type descriptions for the functions which convert between A2A and GenAI `Part`s. It then allows passing instances of those functions to the various A2A-related functions/classes, effectively allowing users to inject their own logic for how part conversion should occur.

The benefit of this pattern is that users can create decorators around the core `Part` conversion logic, which allows them to intercept the cases they care about while delegating the ones they do not to the core converter. This is a pattern we use a lot in the A2A Python SDK.

One example where this type of logic is useful is for extensions: this allows extension logic to, for example, interpret an A2A DataPart into a FunctionResponse using extension-specific logic.

PiperOrigin-RevId: 803186799

tests/unittests/agents/test_remote_a2a_agent.py

@@ -441,7 +441,14 @@ class TestRemoteA2aAgentMessageHandling:
   def setup_method(self):
     """Setup test fixtures."""
     self.agent_card = create_test_agent_card()
-    self.agent = RemoteA2aAgent(name="test_agent", agent_card=self.agent_card)
+    self.mock_genai_part_converter = Mock()
+    self.mock_a2a_part_converter = Mock()
+    self.agent = RemoteA2aAgent(
+        name="test_agent",
+        agent_card=self.agent_card,
+        genai_part_converter=self.mock_genai_part_converter,
+        a2a_part_converter=self.mock_a2a_part_converter,
+    )
 
     # Mock session and context
     self.mock_session = Mock(spec=Session)
@@ -519,20 +526,17 @@ class TestRemoteA2aAgentMessageHandling:
     ) as mock_convert:
       mock_convert.return_value = mock_event
 
-      with patch(
-          "google.adk.agents.remote_a2a_agent.convert_genai_part_to_a2a_part"
-      ) as mock_convert_part:
-        mock_a2a_part = Mock()
-        mock_convert_part.return_value = mock_a2a_part
+      mock_a2a_part = Mock()
+      self.mock_genai_part_converter.return_value = mock_a2a_part
 
-        result = self.agent._construct_message_parts_from_session(
-            self.mock_context
-        )
+      result = self.agent._construct_message_parts_from_session(
+          self.mock_context
+      )
 
-        assert len(result) == 2  # Returns tuple of (parts, context_id)
-        assert len(result[0]) == 1  # parts list
-        assert result[0][0] == mock_a2a_part
-        assert result[1] is None  # context_id
+      assert len(result) == 2  # Returns tuple of (parts, context_id)
+      assert len(result[0]) == 1  # parts list
+      assert result[0][0] == mock_a2a_part
+      assert result[1] is None  # context_id
 
   def test_construct_message_parts_from_session_empty_events(self):
     """Test message parts construction with empty events."""
@@ -575,7 +579,10 @@ class TestRemoteA2aAgentMessageHandling:
 
       assert result == mock_event
       mock_convert.assert_called_once_with(
-          mock_a2a_message, self.agent.name, self.mock_context
+          mock_a2a_message,
+          self.agent.name,
+          self.mock_context,
+          self.mock_a2a_part_converter,
       )
       # Check that metadata was added
       assert result.custom_metadata is not None
@@ -613,7 +620,10 @@ class TestRemoteA2aAgentMessageHandling:
 
       assert result == mock_event
       mock_convert.assert_called_once_with(
-          mock_a2a_task, self.agent.name, self.mock_context
+          mock_a2a_task,
+          self.agent.name,
+          self.mock_context,
+          self.mock_a2a_part_converter,
       )
       # Check that metadata was added
       assert result.custom_metadata is not None
@@ -649,7 +659,14 @@ class TestRemoteA2aAgentExecution:
   def setup_method(self):
     """Setup test fixtures."""
     self.agent_card = create_test_agent_card()
-    self.agent = RemoteA2aAgent(name="test_agent", agent_card=self.agent_card)
+    self.mock_genai_part_converter = Mock()
+    self.mock_a2a_part_converter = Mock()
+    self.agent = RemoteA2aAgent(
+        name="test_agent",
+        agent_card=self.agent_card,
+        genai_part_converter=self.mock_genai_part_converter,
+        a2a_part_converter=self.mock_a2a_part_converter,
+    )
 
     # Mock session and context
     self.mock_session = Mock(spec=Session)