fix(auth): accept RFC 8414 metadata served at the OIDC well-known path

The SDK's `discoverAuthorizationServerMetadata` picks its validation schema
from the well-known filename that resolved rather than from the document that
came back, so anything found at `/.well-known/openid-configuration` is
validated as an OpenID provider document — requiring `jwks_uri`,
`subject_types_supported` and `id_token_signing_alg_values_supported`, three
fields RFC 8414 does not define. RFC 8414 §5 permits that filename for general
OAuth metadata, so a conforming plain OAuth 2.0 authorization server is
rejected; and because the parse throws instead of continuing the candidate
loop, discovery aborts and the connection fails outright.

Filed upstream as modelcontextprotocol/typescript-sdk#2733.

`core/auth/oidcDiscoveryCompat.ts` works around it without fabricating a
field: on a failed RFC 8414 candidate it probes the OIDC candidates the SDK
would try next and, when one returns a document that satisfies
`OAuthMetadataSchema` but fails the OIDC schema, serves that body as the RFC
8414 response so the SDK picks the schema that describes it. A genuine OpenID
provider document is left to the SDK's own OIDC leg. Issuer validation runs
unchanged, since the substituted document is the one the server published.

It wraps `effectiveAuthFetch` — above the fetch tracker, the opposite of
`withOAuthEndpointOverrides` — so the Network tab still records the real 404
and the real probe rather than the substitution.

The candidate derivation mirrors the SDK's `buildDiscoveryUrls`, which a fetch
wrapper cannot call, and a test pins it against that export so the two cannot
drift.

Adds `oauth.asMetadataPath` to the composable test server (mirroring
`oauth.resourceMetadataPath`) and the `oauth-rfc8414-at-oidc-path-http.json`
showcase config that reproduces the failure.

Closes #2172

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UL9retfvAXRgvi6EWk4SY4
Signed-off-by: cliffhall <cliff@futurescale.com>

core/auth/index.ts

@@ -45,6 +45,11 @@ export {
   withOAuthEndpointOverrides,
 } from "./endpointOverrides.js";
 export type { OAuthEndpointOverrides } from "./endpointOverrides.js";
+export {
+  oidcDiscoveryCandidates,
+  isRfc8414OnlyMetadata,
+  withRfc8414OidcCompat,
+} from "./oidcDiscoveryCompat.js";
 
 // Storage
 export type {