fix: resolve circular imports

[daniel-sanche]

We have a couple circular imports in the library:

• _mtls_helper and _agent_identity_utils both require each other
• credentials and _default form a cycle

This makes it difficult to import the library into google3

This PR resolved the import issues

[gemini-code-assist]

Summary of Changes

Hello @daniel-sanche, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses critical circular import issues within the library, specifically between _mtls_helper and _agent_identity_utils, and between credentials and _default. These dependencies were preventing the library from being imported into Google3. By refactoring the call_client_cert_callback function and adjusting logger initialization, the PR successfully breaks these cycles, improving module independence and importability.

Highlights

• Refactored call_client_cert_callback: The call_client_cert_callback function has been moved from google/auth/_agent_identity_utils.py to google/auth/transport/_mtls_helper.py to break a circular dependency.
• Resolved _mtls_helper import cycle: The direct import of _mtls_helper from google/auth/_agent_identity_utils.py has been removed, eliminating a circular import between these two modules.
• Addressed _LOGGER circular import: The google/auth/credentials.py module now directly initializes its _LOGGER instance using the logging module, removing its dependency on google.auth._default for the logger.
• Updated related tests: Corresponding test files have been updated to reflect the function move and import changes, ensuring continued test coverage and correctness.

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[gemini-code-assist]

Code Review

This pull request effectively resolves the circular import issues between _mtls_helper and _agent_identity_utils, and between credentials and _default. The approach of moving the call_client_cert_callback function to _mtls_helper is a clean solution to break the first cycle. Similarly, initializing the logger directly in credentials.py breaks the second cycle while maintaining logging behavior. The test files have been updated accordingly to reflect these refactorings. I've only found a couple of minor style issues regarding missing newlines at the end of files.