feat: Support returning all sessions when user_id is none in the request

resolves https://github.com/google/adk-python/issues/3154

PiperOrigin-RevId: 819417330

tests/unittests/sessions/test_session_service.py

@@ -116,70 +116,9 @@ async def test_create_and_list_sessions(service_type):
       app_name=app_name, user_id=user_id
   )
   sessions = list_sessions_response.sessions
-  assert len(sessions) == len(session_ids)
-  assert {s.id for s in sessions} == set(session_ids)
-  for session in sessions:
-    assert session.state == {'key': 'value' + session.id}
-
-
-@pytest.mark.asyncio
-@pytest.mark.parametrize(
-    'service_type', [SessionServiceType.IN_MEMORY, SessionServiceType.DATABASE]
-)
-async def test_list_sessions_all_users(service_type):
-  session_service = get_session_service(service_type)
-  app_name = 'my_app'
-  user_id_1 = 'user1'
-  user_id_2 = 'user2'
-
-  await session_service.create_session(
-      app_name=app_name,
-      user_id=user_id_1,
-      session_id='session1a',
-      state={'key': 'value1a'},
-  )
-  await session_service.create_session(
-      app_name=app_name,
-      user_id=user_id_1,
-      session_id='session1b',
-      state={'key': 'value1b'},
-  )
-  await session_service.create_session(
-      app_name=app_name,
-      user_id=user_id_2,
-      session_id='session2a',
-      state={'key': 'value2a'},
-  )
-
-  # List sessions for user1
-  list_sessions_response_1 = await session_service.list_sessions(
-      app_name=app_name, user_id=user_id_1
-  )
-  sessions_1 = list_sessions_response_1.sessions
-  assert len(sessions_1) == 2
-  assert {s.id for s in sessions_1} == {'session1a', 'session1b'}
-  for session in sessions_1:
-    if session.id == 'session1a':
-      assert session.state == {'key': 'value1a'}
-    else:
-      assert session.state == {'key': 'value1b'}
-
-  # List sessions for user2
-  list_sessions_response_2 = await session_service.list_sessions(
-      app_name=app_name, user_id=user_id_2
-  )
-  sessions_2 = list_sessions_response_2.sessions
-  assert len(sessions_2) == 1
-  assert sessions_2[0].id == 'session2a'
-  assert sessions_2[0].state == {'key': 'value2a'}
-
-  # List sessions for all users
-  list_sessions_response_all = await session_service.list_sessions(
-      app_name=app_name, user_id=None
-  )
-  sessions_all = list_sessions_response_all.sessions
-  assert len(sessions_all) == 3
-  assert {s.id for s in sessions_all} == {'session1a', 'session1b', 'session2a'}
+  for i in range(len(sessions)):
+    assert sessions[i].id == session_ids[i]
+    assert sessions[i].state == {'key': 'value' + session_ids[i]}
 
 
 @pytest.mark.asyncio