fix(sessions): honor zero recent events in database service

Merge google#5965

## Summary
- honor `GetSessionConfig(num_recent_events=0)` in `DatabaseSessionService`
- add cross-backend regression coverage for zero recent events in `test_get_session_with_config`
- keep existing positive-limit and no-config behavior unchanged

## Motivation
`GetSessionConfig` documents `num_recent_events=0` as returning no events. InMemory and Sqlite already preserve that behavior, and ADK call sites use `num_recent_events=0` when they only need to check whether a session exists. `DatabaseSessionService` used a truthy check, so `0` skipped the SQL `LIMIT` and returned the full event history.

## Testing
- `uv run --extra test pytest tests/unittests/sessions/test_session_service.py::test_get_session_with_config -q`
- `uv run --extra test pytest tests/unittests/sessions -q`
- `uvx pyink --check src/google/adk/sessions/database_session_service.py tests/unittests/sessions/test_session_service.py`
- `uv run python -m py_compile src/google/adk/sessions/database_session_service.py tests/unittests/sessions/test_session_service.py`
- `git diff --check`

COPYBARA_INTEGRATE_REVIEW=google#5965 from White-Mouse:codex/adk-session-zero-events d64df8f
PiperOrigin-RevId: 929438116

Author: White-Mouse

src/google/adk/sessions/database_session_service.py

@@ -531,7 +531,7 @@ async def get_session(
 
       stmt = stmt.order_by(schema.StorageEvent.timestamp.desc())
 
-      if config and config.num_recent_events:
+      if config and config.num_recent_events is not None:
         stmt = stmt.limit(config.num_recent_events)
 
       result = await sql_session.execute(stmt)

tests/unittests/sessions/test_session_service.py

@@ -1058,6 +1058,13 @@ async def test_get_session_with_config(session_service):
   events = session.events
   assert len(events) == num_test_events
 
+  # Explicitly requesting zero recent events should return no event history.
+  config = GetSessionConfig(num_recent_events=0)
+  session = await session_service.get_session(
+      app_name=app_name, user_id=user_id, session_id=session.id, config=config
+  )
+  assert not session.events
+
   # Only expect the most recent 3 events.
   num_recent_events = 3
   config = GetSessionConfig(num_recent_events=num_recent_events)