fix: Enable pool_pre_ping by default for non-SQLite database engines

This change sets `pool_pre_ping=True` in SQLAlchemy engine kwargs for database backends other than SQLite. This helps ensure that connections from the pool are still valid before being used, preventing issues with stale or disconnected connections. Tests are added to verify the default behavior and that explicit overrides are respected

Close #4211

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 864886767

src/google/adk/sessions/database_session_service.py

@@ -113,6 +113,8 @@ class DatabaseSessionService(BaseSessionService):
         connect_args = dict(engine_kwargs.get("connect_args", {}))
         connect_args.setdefault("check_same_thread", False)
         engine_kwargs["connect_args"] = connect_args
+      elif url.get_backend_name() != "sqlite":
+        engine_kwargs.setdefault("pool_pre_ping", True)
 
       db_engine = create_async_engine(db_url, **engine_kwargs)
       if db_engine.dialect.name == "sqlite":