google-auth: Clean up test coverage and outdated docstrings for mTLS and Identity Pool

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.

Start with tests/transport/test_aio_mtls_helper.py and update the docstring and comments for test_get_client_cert_and_key_callback_sync. Then inspect google/auth/compute_engine/_mtls.py and google/auth/identity_pool.py alongside their existing tests, covering MdsMtlsAdapter.close() and the noted RefreshError paths. Run the affected tests and confirm the outdated TypeError references are removed and the missing paths are covered.

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