chore: Lazy import DatabaseSessionService in the adk/sessions/ module

This accounts for about 8% of latency in cold start, because DatabaseSessionService imports from sqlalchemy. After the change, DatabaseSessionService is only imported if it's really needed by the program. The other objects in this file are not affected because they contribute to a small fraction of the latency.

Co-authored-by: Liang Wu <wuliang@google.com>
PiperOrigin-RevId: 829009868

Author: wuliang229

src/google/adk/sessions/__init__.py

@@ -11,31 +11,31 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-import logging
-
 from .base_session_service import BaseSessionService
 from .in_memory_session_service import InMemorySessionService
 from .session import Session
 from .state import State
 from .vertex_ai_session_service import VertexAiSessionService
 
-logger = logging.getLogger('google_adk.' + __name__)
-
-
 __all__ = [
     'BaseSessionService',
+    'DatabaseSessionService',
     'InMemorySessionService',
     'Session',
     'State',
     'VertexAiSessionService',
 ]
 
-try:
-  from .database_session_service import DatabaseSessionService
 
-  __all__.append('DatabaseSessionService')
-except ImportError:
-  logger.debug(
-      'DatabaseSessionService require sqlalchemy>=2.0, please ensure it is'
-      ' installed correctly.'
-  )
+def __getattr__(name: str):
+  if name == 'DatabaseSessionService':
+    try:
+      from .database_session_service import DatabaseSessionService
+
+      return DatabaseSessionService
+    except ImportError as e:
+      raise ImportError(
+          'DatabaseSessionService requires sqlalchemy>=2.0, please ensure it is'
+          ' installed correctly.'
+      ) from e
+  raise AttributeError(f'module {__name__!r} has no attribute {name!r}')