google-auth: Clean up test coverage and outdated docstrings for mTLS and Identity Pool

[nbayati]

There are a few minor gaps in test coverage and test documentation that should be cleaned up in the google-auth library:

• In tests/transport/test_aio_mtls_helper.py, the docstring and comments for test_get_client_cert_and_key_callback_sync reference a TypeError fallback that is no longer used (the code now uses inspect.isawaitable()).
• The MdsMtlsAdapter.close() method in google/auth/compute_engine/_mtls.py lacks test coverage.
• There are several RefreshError exception paths in google/auth/identity_pool.py (specifically around leaf certificate retrieval and parsing) that are currently missing test coverage.