## Motivation and Context
SEP-2351 (modelcontextprotocol/modelcontextprotocol#2351, merged for the 2026-07-28 spec release)
makes explicit that MCP uses the default `oauth-authorization-server` well-known URI suffix from
RFC 8414 Section 3.1 and defines no application-specific suffix.
`MCP::Client::OAuth::Discovery.authorization_server_metadata_urls` already conforms:
it probes the RFC 8414 default suffix first (path-inserted form for path issuers) and
otherwise uses only the OpenID Connect Discovery `openid-configuration` suffix.
Neither the Python SDK nor the TypeScript SDK needed a behavior change for this SEP either;
their tracking issues (python-sdk#2799, typescript-sdk#2256) remain documentation-level.
This change documents the SEP-2351 guarantee in the discovery helper and locks the behavior
in with regression tests so a future refactor cannot silently introduce a non-default suffix.
Resolves#384.
## How Has This Been Tested?
New regression tests in `test/mcp/client/oauth/discovery_test.rb`:
- the first probed candidate for a root issuer is exactly `/.well-known/oauth-authorization-server`
- every generated candidate uses only the `oauth-authorization-server` or `openid-configuration` suffix
- the RFC 8414 path-inserted `oauth-authorization-server` candidate comes first for issuers with a path component
- an issuer URL with a trailing slash is treated as a root issuer
## Breaking Changes
None. Documentation and regression tests only; no behavior change.
2a26ad2030