feat: support private_key_jwt for CIMD clients

Summary

Support private_key_jwt client authentication for clients identified by a Client ID Metadata Document.

Current behavior

CIMD support accepts only token_endpoint_auth_method: "none". Documents that advertise both none and private_key_jwt can work because the provider selects none, but a client that requires private_key_jwt cannot complete authorization code exchange.

This is not a core MCP compliance blocker because the MCP specification makes private_key_jwt optional. It is useful for clients that want stronger authentication than an unauthenticated public client.

Acceptance criteria

• Parse and validate the client's jwks or jwks_uri metadata according to the CIMD draft.
• Authenticate token endpoint requests using private_key_jwt.
• Validate assertion issuer, subject, audience, signature, expiration, and unique identifier/replay constraints.
• Apply SSRF protections to remotely fetched JWKS documents.
• Advertise private_key_jwt only when the complete token-endpoint path is supported.
• Add positive and negative tests for key rotation, invalid audience, expiry, replay, unknown keys, malformed assertions, and SSRF-sensitive URLs.
• Document how this interacts with the global_fetch_strictly_public compatibility flag.

References

• https://modelcontextprotocol.io/specification/2026-07-28/basic/authorization/client-registration#client-id-metadata-documents
• https://datatracker.ietf.org/doc/html/draft-ietf-oauth-client-id-metadata-document-00
• https://datatracker.ietf.org/doc/html/rfc7523