[Dashboard] Add tests to make sure backend change won't break frontend

[jjyao]

What happened + What you expected to happen

Recently we made some backend change that breaks the frontend but it's not caught by our tests: #47714. This reveals that we lack test coverage. This issue tracks the things we need to do to prevent this from happening in the future:

1. Add pydantic model for every API
2. Make sure every API is tested by unit tests
3. Generate client model request code from pydantic model

Versions / Dependencies

master

Reproduction script

N/A

Issue Severity

None

[jjyao]

cc @alanwguo