fix(sessions): apply after_timestamp and num_recent_events together in VertexAiSessionService

Previously, `get_session` sent the server-side `timestamp>=` filter only when `num_recent_events` was unset, so setting both `GetSessionConfig` filters silently dropped the timestamp one.

This CL sends `after_timestamp` to the server whenever it is set and still trims to `num_recent_events` client-side, so both filters now compose.

Every other ADK session service already applies both filters together -- the in-memory, database and sqlite services here, and the Vertex services in the Go/JS/Kotlin/Java ports; this was the last one out of parity.

PiperOrigin-RevId: 952818684

Author: google-genai-bot

src/google/adk/sessions/vertex_ai_session_service.py

@@ -236,7 +236,7 @@ async def get_session(
     async with self._get_api_client() as api_client:
       # Get session resource and events in parallel.
       list_events_kwargs = {}
-      if config and not config.num_recent_events and config.after_timestamp:
+      if config and config.after_timestamp:
         # Filter events based on timestamp.
         list_events_kwargs['config'] = {
             'filter': 'timestamp>="{}"'.format(

tests/unittests/sessions/test_vertex_ai_session_service.py

@@ -796,6 +796,26 @@ async def test_get_session_with_after_timestamp_filter():
   assert session.events[0].id == '456'
 
 
+@pytest.mark.asyncio
+@pytest.mark.usefixtures('mock_get_api_client')
+async def test_get_session_with_num_recent_events_and_after_timestamp():
+  session_service = mock_vertex_ai_session_service()
+  session = await session_service.get_session(
+      app_name='123',
+      user_id='user',
+      session_id='2',
+      config=GetSessionConfig(
+          num_recent_events=2,
+          after_timestamp=isoparse('2024-12-12T12:12:13.0Z').timestamp(),
+      ),
+  )
+  assert session is not None
+  # after_timestamp must be applied even though num_recent_events is set;
+  # without it both events would be returned.
+  assert len(session.events) == 1
+  assert session.events[0].id == '456'
+
+
 @pytest.mark.asyncio
 @pytest.mark.usefixtures('mock_get_api_client')
 async def test_get_session_keeps_events_newer_than_update_time(