Discover protected-resource metadata, falling back to treating the MCP
server's origin as the issuer when no PRM document exists (the pre-RFC-9728
2025-03-26 behavior). Returns the issuer to use for AS discovery, and sets
fromProtectedResourceMetadata to true on the modern RFC 9728 path or false
on the 2025-03-26 origin fallback (the issuer enforcement implications of each
path are documented on bindDiscoveredIssuer).
Discover protected-resource metadata, falling back to treating the MCP server's origin as the issuer when no PRM document exists (the pre-RFC-9728 2025-03-26 behavior). Returns the issuer to use for AS discovery, and sets fromProtectedResourceMetadata to true on the modern RFC 9728 path or false on the 2025-03-26 origin fallback (the issuer enforcement implications of each path are documented on bindDiscoveredIssuer).