Merge https://github.com/google/adk-python/pull/4435
### Link to Issue or Description of Change
- Closes: #4302
**Problem:**
`VertexAiSessionService.list_sessions()` only returns the first ~100 sessions. The `sessions_iterator` from `api_client.agent_engines.sessions.list()` is an `AsyncPager` — it implements `__aiter__`/`__anext__` for fetching subsequent pages, but the code uses a plain `for` loop which only calls `__iter__`/`__next__`, so it never fetches beyond the first page.
**Solution:**
Changed `for api_session in sessions_iterator` to `async for api_session in sessions_iterator` so the `AsyncPager` actually paginates. Updated the test mock to return an `AsyncIterableList` (supports both sync and async iteration) instead of a bare list, so the tests properly simulate real `AsyncPager` behaviour.
### Testing Plan
**Unit Tests:**
```
$ pytest tests/unittests/sessions/
115 passed, 1 warning in 2.25s
```
The existing `test_list_sessions`, `test_list_sessions_with_pagination`, and `test_list_sessions_all_users` all continue to pass with the updated mock.
Co-authored-by: Liang Wu <wuliang@google.com>
COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/4435 from anmolg1997:fix/vertex-ai-session-service-pagination 14c71b607ecbf2215f4b9ba6eb4b0ff6b9eaf740
PiperOrigin-RevId: 868466166
