(before/after) agent callbacks are invoked throughout the provided chain until one callback does not return None. Callbacks can be async and sync.
PiperOrigin-RevId: 756359693
* Move unit tests to root package.
* Adds deps to "test" extra, and mark two broken tests in tests/unittests/auth/test_auth_handler.py
* Adds github workflow
* minor fix in lite_llm.py for python 3.9.
* format pyproject.toml
