fix(sessions): respect num_recent_events=0 in DatabaseSessionService

`DatabaseSessionService.get_session` used a truthy check
(`if config and config.num_recent_events:`) which treated `0` as
"no limit" and returned all events, contradicting the documented
behavior in `GetSessionConfig` ("if 0, no events are returned").

Switch to `is not None` so that `LIMIT 0` is appended for the zero
case. Add a boundary assertion to `test_get_session_with_config`,
which is parametrized over all four session backends (InMemory,
InMemory-light-copy, Database, Sqlite).

Closes #5730

Author: tiino

src/google/adk/sessions/database_session_service.py

@@ -520,7 +520,7 @@ 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

@@ -1067,6 +1067,13 @@ async def test_get_session_with_config(session_service):
   assert len(events) == num_recent_events
   assert events[0].timestamp == num_test_events - num_recent_events + 1
 
+  # num_recent_events=0 should return no events (boundary case).
+  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 session.events == []
+
   # Only expect events after timestamp 4.0 (inclusive), i.e., 2 events.
   after_timestamp = 4.0
   config = GetSessionConfig(after_timestamp=after_timestamp)