When GITHUB_APP_ID/INSTALLATION_ID/PRIVATE_KEY_FILE are unset (Gitea-
canonical deployment or suspended GitHub App org), generateAppInstallation
Token() returns "required" — a permanent configuration error, not a
transient one. Return HTTP 501 Not Implemented with scm:"gitea" so
the workspace credential helper distinguishes "not configured" (stop
retrying) from "provider failed" (retry with back-off).
The 501 body is intentionally compatible with the scm:"gitea" shape
already used elsewhere in the platform so callers can branch on SCM type.