Implement server-side support for Client ID Metadata Documents (CIMD)

Summary

PR #1652 implemented client-side support for Client ID Metadata Documents (CIMD) per SEP-991, but the server-side implementation is missing. Authorization servers built with the Python SDK cannot currently support CIMD.

Background

CIMD (draft-ietf-oauth-client-id-metadata-document-00) allows OAuth clients to use HTTPS URLs as client identifiers, where the URL points to a JSON document containing client metadata. This is the recommended registration approach per the MCP spec (ahead of DCR).

From the MCP Authorization spec:

Authorization servers and MCP clients SHOULD support OAuth Client ID Metadata Documents

Current State

Client-side (implemented in #1652)

• ✅ is_valid_client_metadata_url() - validates HTTPS URLs with path component
• ✅ should_use_client_metadata_url() - checks if server advertises CIMD support
• ✅ create_client_info_from_metadata_url() - uses URL as client_id
• ✅ OAuth flow integration with DCR fallback

Server-side (missing)

• ❌ Never advertises client_id_metadata_document_supported=true in OAuth metadata
• ❌ No detection of URL-formatted client_ids
• ❌ No metadata document fetching
• ❌ No validation (client_id matching, redirect_uri verification, document structure)
• ❌ No caching infrastructure
• ❌ No SSRF protection for fetching

Spec Requirements for Authorization Servers

From the MCP spec and CIMD RFC:

References

• MCP Spec: https://modelcontextprotocol.io/specification/2025-11-25/basic/authorization#client-id-metadata-documents
• CIMD RFC: https://datatracker.ietf.org/doc/html/draft-ietf-oauth-client-id-metadata-document-00
• SEP-991: https://github.com/modelcontextprotocol/modelcontextprotocol/issues/991
• Client-side PR: #1652
• Original tracking issue: #1538 (closed prematurely - only client-side was implemented)

_{AI Disclaimer}

Empieza revisando la compatibilidad con CIMD en el cliente en PR #1652, así como los metadatos de OAuth del SDK y el manejo de la autorización en el servidor. Relaciona la detección de URL client_id ausente, la obtención de documentos, la validación, el almacenamiento en caché y las protecciones contra SSRF con los puntos de entrada existentes. Se considera completado cuando los servidores de autorización anuncian compatibilidad con CIMD y cumplen los requisitos indicados de CIMD y MCP sin almacenar en caché los errores.