`/mcp reauth` and `/mcp add` consume `authResult.oauth` directly when the JSON error body carries endpoints, skipping `discoverOAuthEndpoints`. Leaving `oauth.scopes` empty there meant a challenge-only `scope="…"` still yielded a scope-less grant even though `AuthDetectionResult.scopes` recorded it.
Merge `challengeScopes` into the returned `OAuthEndpoints` inside `analyzeAuthError` so every consumer sees the same scope. Added a regression test where the JSON body advertises endpoints without `scopes` and the challenge is the only source.
Refs #4467