Broaden authorization server discovery to include OIDC fallbacks

[sonmaximum]

Summary

Extend build_oauth_authorization_server_metadata_discovery_urls so that the MCP client always probes both OAuth and OpenID discovery endpoints, with path-aware variants and deduplication. This lets the client interoperate with authorization servers that only expose /.well-known/openid-configuration.

Change Details

• Use auth_server_url or server_url as a single source of truth, so we always consider the configured base when no metadata URL is advertised.
• Populate discovery attempts with:
  • Path-aware OAuth metadata
  • Path-aware OIDC metadata (/.well-known/openid-configuration{path})
  • Nested-path OIDC ({path}/.well-known/openid-configuration)
  • Root-level OAuth and OIDC fallbacks
• Deduplicate the list to avoid redundant fetches when different branches yield the same URL.

Motivation and Context

• Some IdPs only publish OIDC discovery. Without trying those endpoints, the OAuth flow fails even though the IdP is spec compliant.
• RFC 8414 explicitly recommends path-aware fallbacks for both OAuth and OIDC discovery; the existing implementation only covers part of that matrix.
• Deduping avoids duplicated network calls with path == "/" or when config already points at a well-known URL.

How Has This Been Tested?

• Verified URL list generation manually for:
  • No resource_metadata, base URL https://idp.example.com -> probes both OAuth and OIDC root well-known endpoints.
  • Path-based auth_server_url like https://idp.example.com/tenantA -> probes path-aware OAuth/OIDC endpoints plus root fallbacks without duplicates.
• Updated existing tests with updated behavior and added new tests to covered broadened case coverage. Ensured pytest tests pass.

Types of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
  (either depending on perspective?)

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

See related issue #1785