[WIP] Update gcp-auth-extension module to follow style guide

[Copilot]

Update gcp-auth-extension module to follow style guide recommendations:

• Explore repository structure and understand current code
• Run tests and verify current state (all tests passing)
• Run spotless check to verify formatting (passes)
• Add static imports for commonly used methods and constants
• Make public non-internal classes final where appropriate
• Ensure test classes are package-protected (already correct)
• Verify field finality and class organization
• Run tests after changes to ensure no regressions
• Apply spotless formatting

Key changes needed:

1. Add static imports for AttributeKey.*, Collections.*, Collectors.*, Arrays.stream, Locale.*
2. Make GoogleAuthException class final (public non-internal class)
3. Verify class organization follows recommended order
4. Test all changes for regression

Fixes #379.

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