Implement per-service protected resource metadata (RFC 9728)

[dgellow]

401 responses now include service-specific metadata URIs that advertise the correct per-service resource, so clients request tokens with the right audience claim.

[dgellow]

/gemini review

[gemini-code-assist]

Summary of Changes

Hello @dgellow, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the OAuth 2.0 implementation by introducing support for per-service protected resource metadata, as specified in RFC 9728. This change ensures that clients can accurately discover the appropriate resource URIs for individual services, enabling precise audience claim validation in OAuth tokens. The system now dynamically provides service-specific metadata in 401 responses, guiding clients to request tokens tailored to the specific protected resource they are trying to access.

Highlights

• Per-Service Protected Resource Metadata: New endpoints have been introduced at /.well-known/oauth-protected-resource/{service} to provide OAuth 2.0 Protected Resource Metadata specific to individual services, aligning with RFC 9728 Section 5.2.
• Dynamic WWW-Authenticate Headers: 401 responses now dynamically include service-specific resource_metadata URIs in the WWW-Authenticate header, guiding clients to request tokens with the correct audience claims as per RFC 8707.
• Deprecated Generic Endpoint: The generic /.well-known/oauth-protected-resource endpoint now returns a 404 Not Found, directing clients to use the new per-service endpoints for metadata discovery.
• Service Name Extraction: A new utility function, ExtractServiceNameFromPath, has been added to reliably determine the service name from a request path, supporting dynamic metadata generation.

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[gemini-code-assist]

Code Review

This pull request effectively implements per-service protected resource metadata as per RFC 9728. The changes are well-structured, with clear separation of concerns between metadata generation, routing, and token validation. The addition of comprehensive unit and integration tests ensures the new functionality is robust and covers various scenarios, including error cases. The deprecation of old functions and handlers is also handled well with clear comments. I've found one area for improvement in the service name extraction logic to make it more robust against request paths that don't match the issuer's base path. Overall, this is a high-quality contribution.