feat(auth): ✨ implement PKCE security and endpoint fallback strategies

[Mirrowel]

• Integrate PKCE (Proof Key for Code Exchange) into the OAuth flow in GoogleOAuthBase, including state parameter validation for enhanced security.
• Implement robust endpoint fallback chains (Production ↔ Daily Sandbox) in AntigravityAuthBase for project discovery and user onboarding.
• Update User-Agent and X-Goog-Api-Client headers to align with recent Node.js client versions.
• Refactor project ID extraction and onboarding polling logic to handle variable API response formats and errors more gracefully.

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Important

Implement PKCE and endpoint fallback strategies in OAuth flows for enhanced security and reliability.

• Security Enhancements:
  • Implement PKCE in GoogleOAuthBase for OAuth flow, including state parameter validation.
  • Add _generate_pkce(), _encode_oauth_state(), and _decode_oauth_state() methods.
• Endpoint Fallback:
  • Add endpoint fallback strategies in AntigravityAuthBase for loadCodeAssist and onboardUser.
  • Use ANTIGRAVITY_LOAD_ENDPOINT_ORDER and ANTIGRAVITY_ENDPOINT_FALLBACKS from gemini_shared_utils.py.
• Header Updates:
  • Update User-Agent and X-Goog-Api-Client headers in antigravity_auth_base.py and google_oauth_base.py.
• Refactoring:
  • Refactor project ID extraction in AntigravityAuthBase to handle variable API response formats.

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