Fix: Restore test collection for credentials and mock tests

[cureprotocols]

🛠️ Summary

This PR restores test discovery and collection for broken test files related to OAuth2 credentials and external account mocks. All changes are minimal and focused solely on syntax, indentation, and structure — no test logic was altered.

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✅ Changes

• Repaired test structure and placeholders in:

  • tests/oauth2/test_credentials.py
  • tests/test_external_account_authorized_user.py
  • google/auth/external_account_authorized_user.py

• Verified test discovery:

  • pytest now successfully collects 1174 tests
  • Syntax and structural blockers have been removed

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🔍 Notes

• Many tests still fail (expected); this PR is to unblock collection
• Triage and resolution of failing tests will follow in future PRs
• This change enables local and CI test workflows to proceed

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