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

[Copilot]

Updating gcp-auth-extension module to follow style guide requirements. Analysis complete, implementing minimal changes:

• Analyze current code against style guide requirements
• Run existing tests to ensure code is working
• Check spotless formatting (already compliant)
• Add final keyword to public classes where appropriate
• Add static imports for commonly used utility methods
• Add defensive null checks where appropriate
• Run tests to ensure changes don't break functionality
• Run spotless to ensure formatting compliance

Key changes identified:

1. Make GcpAuthAutoConfigurationCustomizerProvider class final
2. Make GoogleAuthException class final
3. Add static imports for Collectors.*, Arrays.*, Level.*, Objects.requireNonNull
4. Add defensive null checks using requireNonNull

Fixes #250.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.