fix(sessions): use async iteration for VertexAiSessionService.list_sessions pagination

The sessions_iterator returned by api_client.agent_engines.sessions.list()
is an AsyncPager that only fetches additional pages when consumed via
async iteration (__aiter__/__anext__). Using a synchronous `for` loop
only retrieved the first page (~100 sessions), silently dropping all
subsequent pages.

Changed `for` to `async for` to enable proper async pagination,
allowing all sessions to be returned regardless of count.

Fixes #4302

Author: anmolg1997

src/google/adk/sessions/vertex_ai_session_service.py

@@ -215,7 +215,7 @@ async def list_sessions(
           config=config,
       )
 
-      for api_session in sessions_iterator:
+      async for api_session in sessions_iterator:
         sessions.append(
             Session(
                 app_name=app_name,

tests/unittests/sessions/test_vertex_ai_session_service.py

@@ -264,6 +264,17 @@ async def to_async_iterator(data):
     yield item
 
 
+class AsyncIterableList(list):
+  """A list that also supports async iteration, mimicking AsyncPager behavior."""
+
+  def __aiter__(self):
+    return self._async_iter()
+
+  async def _async_iter(self):
+    for item in self:
+      yield item
+
+
 class MockAsyncClient:
   """Mocks the API Client."""
 
@@ -300,20 +311,20 @@ async def _list_sessions(self, name: str, config: dict[str, Any]):
     if user_id_match:
       user_id = user_id_match.group(1)
       if user_id == 'user_with_pages':
-        return [
+        return AsyncIterableList([
             _convert_to_object(MOCK_SESSION_JSON_PAGE1),
             _convert_to_object(MOCK_SESSION_JSON_PAGE2),
-        ]
-      return [
+        ])
+      return AsyncIterableList([
           _convert_to_object(session)
           for session in self.session_dict.values()
           if session['user_id'] == user_id
-      ]
+      ])
 
     # No user filter, return all sessions
-    return [
+    return AsyncIterableList([
         _convert_to_object(session) for session in self.session_dict.values()
-    ]
+    ])
 
   async def _delete_session(self, name: str):
     session_id = name.split('/')[-1]