Treat warnings as errors in testing

[parthea]

Once the warnings are addressed that appear in build logs, we should enable warnings as errors to ensure that issues are addressed promptly.

The current warnings are for the protobuf dependency which is tracked here

========================================================================================================================= warnings summary ==========================================================================================================================
tests/unit/test_google_api_error_reason.py::test_error_reason
  <frozen importlib._bootstrap>:488: DeprecationWarning: Type google._upb._message.MessageMapContainer uses PyType_Spec with a metaclass that has custom tp_new. This is deprecated and will no longer be allowed in Python 3.14.

tests/unit/test_google_api_error_reason.py::test_error_reason
  <frozen importlib._bootstrap>:488: DeprecationWarning: Type google._upb._message.ScalarMapContainer uses PyType_Spec with a metaclass that has custom tp_new. This is deprecated and will no longer be allowed in Python 3.14.

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html

[parthea]

Transferring this issue to google-cloud-python as the code for python-api-common-protos will be migrated there.