fix: fix httpx client closure during event pagination

Merge https://github.com/google/adk-python/pull/3756

move event iteration inside api_client context in get_session

Move event iteration inside the api_client context manager in VertexAiSessionService.get_session() to prevent client closure during multi-page event fetching.

**Please ensure you have read the [contribution guide](https://github.com/google/adk-python/blob/main/CONTRIBUTING.md) before creating a pull request.**

### Link to Issue or Description of Change

**1. Link to an existing issue (if applicable):**

- Closes: #3757

**2. Or, if no issue exists, describe the change:**

**Problem:**

When a session contains more than 100 events (requiring pagination), `VertexAiSessionService.get_session()` fails with:

```
RuntimeError: Cannot send a request, as the client has been closed.
```

The root cause is that the `events_iterator` is consumed **outside** the `async with self._get_api_client() as api_client:` context block. When the iterator needs to fetch page 2, 3, etc., the API client has already been closed because the `async with` block has exited.

```python
# Current buggy flow:
async with self._get_api_client() as api_client:
    get_session_response, events_iterator = await asyncio.gather(...)
# ← Client closed here

async for event in events_iterator:  # ← Fails on page 2+ (client closed)
    session.events.append(...)
```

**Solution:**

Move the session creation, user validation, and event iteration **inside** the `async with` block so the API client remains open during the entire pagination process:

```python
async with self._get_api_client() as api_client:
    get_session_response, events_iterator = await asyncio.gather(...)
    # Validation and session creation...
    async for event in events_iterator:  # ← Now works for all pages
        session.events.append(...)
# Client closed after all events are fetched
```

### Testing Plan

**Unit Tests:**

- [x] I have added or updated unit tests for my change.
- [x] All unit tests pass locally.

```bash
pytest tests/unittests/sessions/test_vertex_ai_session_service.py -v
```

**Added regression test:** `test_get_session_pagination_keeps_client_open`
- Creates a `MockAsyncClientWithPagination` that tracks whether it's inside the `async with` context
- Raises `RuntimeError` if iteration happens outside the context (matching real httpx behavior)
- Simulates 3 pages of events (100 + 100 + 50 = 250 events)
- Verifies all 250 events are successfully retrieved

**Manual End-to-End (E2E) Tests:**

1. Deploy an ADK agent to Vertex AI Agent Engine
2. Create a session and send 100+ messages to accumulate >100 events
3. Verify `get_session()` successfully retrieves all events without error

**Before fix:**
```
RuntimeError: Cannot send a request, as the client has been closed.
```

**After fix:**
- Session with 201 events (3 pages) loads successfully
- All events are retrieved and appended to the session

### Checklist

- [x] I have read the [CONTRIBUTING.md](https://github.com/google/adk-python/blob/main/CONTRIBUTING.md) document.
- [x] I have performed a self-review of my own code.
- [x] I have commented my code, particularly in hard-to-understand areas.
- [x] I have added tests that prove my fix is effective or that my feature works.
- [x] New and existing unit tests pass locally with my changes.
- [x] I have manually tested my changes end-to-end.
- [x] Any dependent changes have been merged and published in downstream modules.

### Additional context

This bug affects any production deployment where users have extended conversations. Sessions accumulating >100 events (which triggers pagination) become completely unusable as the agent cannot load the session to process new messages.

The fix is minimal and maintains backward compatibility - it only changes the scope of the `async with` block without altering any logic or return values.

**Affected versions:** Tested on google-adk 1.19.0, but the bug exists in earlier versions as well.

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/3756 from AlexisMarasigan:fix/vertex-ai-session-service-paginatio 01fbafa6524312f24f7c9feaffb07bff0ad49b77
PiperOrigin-RevId: 855451813

src/google/adk/sessions/vertex_ai_session_service.py

@@ -162,7 +162,6 @@ class VertexAiSessionService(BaseSessionService):
               **list_events_kwargs,
           ),
       )
-
       if get_session_response.user_id != user_id:
         raise ValueError(
             f'Session {session_id} does not belong to user {user_id}.'

tests/unittests/sessions/test_vertex_ai_session_service.py

@@ -397,6 +397,103 @@ class MockAsyncClient:
       self.event_dict[session_id] = ([event_json], None)
 
 
+class MockAsyncClientWithPagination:
+  """Mock client that simulates pagination requiring an open client connection.
+
+  This mock tracks whether the client context is active and raises RuntimeError
+  if iteration occurs outside the context, simulating the real httpx behavior.
+  """
+
+  def __init__(self, session_data: dict, events_pages: list[list[dict]]):
+    self._session_data = session_data
+    self._events_pages = events_pages
+    self._context_active = False
+    self.agent_engines = mock.AsyncMock()
+    self.agent_engines.sessions.get.side_effect = self._get_session
+    self.agent_engines.sessions.events.list.side_effect = self._list_events
+
+  async def __aenter__(self):
+    self._context_active = True
+    return self
+
+  async def __aexit__(self, exc_type, exc_val, exc_tb):
+    self._context_active = False
+
+  async def _get_session(self, name: str):
+    return _convert_to_object(self._session_data)
+
+  async def _list_events(self, name: str, **kwargs):
+    return self._paginated_events_iterator()
+
+  async def _paginated_events_iterator(self):
+    for page in self._events_pages:
+      for event in page:
+        if not self._context_active:
+          raise RuntimeError(
+              'Cannot send a request, as the client has been closed.'
+          )
+        yield _convert_to_object(event)
+
+
+def _generate_events_for_page(session_id: str, start_idx: int, count: int):
+  events = []
+  start_time = isoparse('2024-12-12T12:12:12.123456Z')
+  for i in range(count):
+    idx = start_idx + i
+    event_time = start_time + datetime.timedelta(microseconds=idx * 1000)
+    events.append({
+        'name': (
+            'projects/test-project/locations/test-location/'
+            f'reasoningEngines/123/sessions/{session_id}/events/{idx}'
+        ),
+        'invocation_id': f'invocation_{idx}',
+        'author': 'pagination_user',
+        'timestamp': event_time.isoformat().replace('+00:00', 'Z'),
+    })
+  return events
+
+
+@pytest.mark.asyncio
+async def test_get_session_pagination_keeps_client_open():
+  """Regression test: event iteration must occur inside the api_client context.
+
+  This test verifies that get_session() keeps the API client open while
+  iterating through paginated events. Before the fix, the events_iterator
+  was consumed outside the async with block, causing RuntimeError when
+  fetching subsequent pages.
+  """
+  session_data = {
+      'name': (
+          'projects/test-project/locations/test-location/'
+          'reasoningEngines/123/sessions/pagination_test'
+      ),
+      'update_time': '2024-12-12T12:12:12.123456Z',
+      'user_id': 'pagination_user',
+  }
+  page1_events = _generate_events_for_page('pagination_test', 0, 100)
+  page2_events = _generate_events_for_page('pagination_test', 100, 100)
+  page3_events = _generate_events_for_page('pagination_test', 200, 50)
+
+  mock_client = MockAsyncClientWithPagination(
+      session_data=session_data,
+      events_pages=[page1_events, page2_events, page3_events],
+  )
+
+  session_service = mock_vertex_ai_session_service()
+
+  with mock.patch.object(
+      session_service, '_get_api_client', return_value=mock_client
+  ):
+    session = await session_service.get_session(
+        app_name='123', user_id='pagination_user', session_id='pagination_test'
+    )
+
+  assert session is not None
+  assert len(session.events) == 250
+  assert session.events[0].invocation_id == 'invocation_0'
+  assert session.events[249].invocation_id == 'invocation_249'
+
+
 def mock_vertex_ai_session_service(
     project: Optional[str] = 'test-project',
     location: Optional[str] = 'test-location',