feat: Add support for passing request metadata in RemoteA2AAgent

This change updates `RemoteA2AAgent` to extract and forward custom metadata from session events to the `a2a-sdk`'s `send_message` method. The metadata is looked for under the key `A2A_METADATA_PREFIX + "metadata"` within the `custom_metadata` of the relevant session events. The `a2a-sdk` dependency is also updated to a version that supports this feature.

This feature was added in v0.3.11 of the a2a-sdk library: https://github.com/a2aproject/a2a-python/releases/tag/v0.3.11

PiperOrigin-RevId: 831120978

src/google/adk/agents/remote_a2a_agent.py

@@ -318,7 +318,7 @@ class RemoteA2aAgent(BaseAgent):
 
   def _create_a2a_request_for_user_function_response(
       self, ctx: InvocationContext
-  ) -> Optional[A2AMessage]:
+  ) -> tuple[Optional[A2AMessage], Optional[dict[str, Any]]]:
     """Create A2A request for user function response if applicable.
 
     Args:
@@ -328,34 +328,38 @@ class RemoteA2aAgent(BaseAgent):
       SendMessageRequest if function response found, None otherwise
     """
     if not ctx.session.events or ctx.session.events[-1].author != "user":
-      return None
+      return None, None
     function_call_event = find_matching_function_call(ctx.session.events)
     if not function_call_event:
-      return None
+      return None, None
 
     a2a_message = convert_event_to_a2a_message(
         ctx.session.events[-1], ctx, Role.user, self._genai_part_converter
     )
+    message_metadata = None
     if function_call_event.custom_metadata:
       metadata = function_call_event.custom_metadata
       a2a_message.task_id = metadata.get(A2A_METADATA_PREFIX + "task_id")
       a2a_message.context_id = metadata.get(A2A_METADATA_PREFIX + "context_id")
+      message_metadata = metadata.get(A2A_METADATA_PREFIX + "metadata")
 
-    return a2a_message
+    return a2a_message, message_metadata
 
   def _construct_message_parts_from_session(
       self, ctx: InvocationContext
-  ) -> tuple[list[A2APart], Optional[str]]:
+  ) -> tuple[list[A2APart], Optional[str], Optional[dict[str, Any]]]:
     """Construct A2A message parts from session events.
 
     Args:
       ctx: The invocation context
 
     Returns:
-      List of A2A parts extracted from session events, context ID
+      List of A2A parts extracted from session events, context ID,
+      request metadata
     """
     message_parts: list[A2APart] = []
     context_id = None
+    request_metadata = None
 
     events_to_process = []
     for event in reversed(ctx.session.events):
@@ -365,6 +369,7 @@ class RemoteA2aAgent(BaseAgent):
         if event.custom_metadata:
           metadata = event.custom_metadata
           context_id = metadata.get(A2A_METADATA_PREFIX + "context_id")
+          request_metadata = metadata.get(A2A_METADATA_PREFIX + "metadata")
         break
       events_to_process.append(event)
 
@@ -385,7 +390,7 @@ class RemoteA2aAgent(BaseAgent):
         else:
           logger.warning("Failed to convert part to A2A format: %s", part)
 
-    return message_parts, context_id
+    return message_parts, context_id, request_metadata
 
   async def _handle_a2a_response(
       self, a2a_response: A2AClientEvent | A2AMessage, ctx: InvocationContext
@@ -493,10 +498,12 @@ class RemoteA2aAgent(BaseAgent):
       return
 
     # Create A2A request for function response or regular message
-    a2a_request = self._create_a2a_request_for_user_function_response(ctx)
+    a2a_request, request_metadata = (
+        self._create_a2a_request_for_user_function_response(ctx)
+    )
     if not a2a_request:
-      message_parts, context_id = self._construct_message_parts_from_session(
-          ctx
+      message_parts, context_id, request_metadata = (
+          self._construct_message_parts_from_session(ctx)
       )
 
       if not message_parts:
@@ -522,7 +529,8 @@ class RemoteA2aAgent(BaseAgent):
 
     try:
       async for a2a_response in self._a2a_client.send_message(
-          request=a2a_request
+          request=a2a_request,
+          request_metadata=request_metadata,
       ):
         logger.debug(build_a2a_response_log(a2a_response))