fix(a2a): respect auto_create_session flag in A2A executor session preparation

Author: nickchecan

src/google/adk/a2a/executor/a2a_agent_executor.py

@@ -328,25 +328,11 @@ async def _prepare_session(
       run_request: AgentRunRequest,
       runner: Runner,
   ):
-
-    session_id = run_request.session_id
-    # create a new session if not exists
-    user_id = run_request.user_id
-    session = await runner.session_service.get_session(
-        app_name=runner.app_name,
-        user_id=user_id,
-        session_id=session_id,
+    session = await runner._get_or_create_session(
+        user_id=run_request.user_id,
+        session_id=run_request.session_id,
     )
-    if session is None:
-      session = await runner.session_service.create_session(
-          app_name=runner.app_name,
-          user_id=user_id,
-          state={},
-          session_id=session_id,
-      )
-      # Update run_request with the new session_id
-      run_request.session_id = session.id
-
+    run_request.session_id = session.id
     return session
 
   def _check_new_version_extension(self, context: RequestContext):

src/google/adk/a2a/executor/a2a_agent_executor_impl.py

@@ -281,25 +281,14 @@ async def _resolve_session(
       run_request: AgentRunRequest,
       runner: Runner,
   ):
-    session_id = run_request.session_id
-    # create a new session if not exists
-    user_id = run_request.user_id
-    session = await runner.session_service.get_session(
-        app_name=runner.app_name,
-        user_id=user_id,
-        session_id=session_id,
-        # Checking existence doesn't require event history.
-        config=base_session_service.GetSessionConfig(num_recent_events=0),
+    session = await runner._get_or_create_session(
+        user_id=run_request.user_id,
+        session_id=run_request.session_id,
+        get_session_config=base_session_service.GetSessionConfig(
+            num_recent_events=0
+        ),
     )
-    if session is None:
-      session = await runner.session_service.create_session(
-          app_name=runner.app_name,
-          user_id=user_id,
-          state={},
-          session_id=session_id,
-      )
-      # Update run_request with the new session_id
-      run_request.session_id = session.id
+    run_request.session_id = session.id
 
   def _get_invocation_metadata(
       self, executor_context: ExecutorContext