fix: make DatabaseSessionService visible in API docs

Fixes #4331

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 933310174

Author: GWeale

src/google/adk/sessions/__init__.py

@@ -55,3 +55,7 @@ def __getattr__(name: str):
       raise missing_extra('sqlalchemy', 'db') from e
     return vars(module)['DatabaseSessionService']
   raise AttributeError(f'module {__name__!r} has no attribute {name!r}')
+
+
+def __dir__() -> list[str]:
+  return sorted(__all__)

tests/unittests/sessions/test_session_service.py

@@ -1784,3 +1784,15 @@ async def test_vertex_ai_session_service_raises_not_implemented_for_get_user_sta
   service = VertexAiSessionService(project='proj', location='us-central1')
   with pytest.raises(NotImplementedError):
     await service.get_user_state(app_name='my_app', user_id='u1')
+
+
+def test_database_session_service_visible_in_module_namespace():
+  """DatabaseSessionService must be in dir() so Sphinx autodoc renders it.
+
+  It is imported lazily via module __getattr__, so without an explicit
+  __dir__ it drops out of the generated API reference (issue #4331).
+  """
+  import google.adk.sessions as sessions_module
+
+  assert 'DatabaseSessionService' in dir(sessions_module)
+  assert sessions_module.DatabaseSessionService is DatabaseSessionService