VertexAiSessionService list sessions always returns empty list

[dpark27]

Describe the bug
When using VertexAiSessionService, the list_sessions function always returns an empty result

To Reproduce

1. Initialize a VertexAiSessionService
2. Create a session
3. run session_service.list_sessions(app_name=session.app_name, user_id=your_user_id)

Expected behavior
Return a list of sessions that exist for the user

Desktop (please complete the following information):

• OS: OS X
• Python version(python -V): 3.11.5
• ADK version(pip show google-adk): v0.5.0

Additional context
Add any other context about the problem here.

[DeanChensj]

Hi @dpark27 , what user_id are you using? Is it a special character?

[dpark27]

We are using email addresses as usernames, ie ***@***.***

[DeanChensj]

Thanks for fixing the issue. #996 is now merged.