Check gcloud context_aware/use_client_certificate for mTLS

[google-labs-jules]

Implemented an additional check for mTLS client certificate usage.

• Modified google/auth/_cloud_sdk.py:

  • Added get_context_aware_use_client_certificate() to query gcloud config get context_aware/use_client_certificate.
  • This function handles platform differences (Windows vs. Posix) and suppresses stderr.

• Modified google/auth/transport/_mtls_helper.py:

  • Updated check_use_client_cert() to call the new _cloud_sdk helper if other checks (environment variables) do not result in True.
  • This effectively implements an OR condition: Env Var Check OR Config File Check OR Gcloud Config Check.

• Added/Updated Tests:

  • tests/test__cloud_sdk.py: Added tests for get_context_aware_use_client_certificate.
  • tests/transport/test__mtls_helper.py: Updated tests to cover the new fallback logic and mock the gcloud check.

---

PR created automatically by Jules for task 16361868464918155708 started by @amtk3

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.