surface eval errors + disambiguate auth failures #minor

[BradenBug]

Summary

Batch 2 (service side) of lab-deliverable-framework production readiness. Stops the deployed service hiding actionable, non-secret information from labs: real error messages on the legacy 500 path, and the six distinct auth-failure reasons surfaced as distinct, non-secret messages instead of one opaque 401.

Changes

• Legacy 500 → real message. The legacy /evaluate-response/ + /final-score/ global handler now returns {"detail": "Evaluation failed", "errors": [str(exc)]} (mirroring the existing /v1 errors[]), with the traceback kept server-side only. Withheld from trial tenants (fail-closed: an unknown/unset tenant also withholds).
• 401 disambiguation. resolve_descope_tenant now returns a typed AuthResult(tenant, failure); the middleware + websocket auth map each AuthFailure to a distinct status+detail — allowlist-miss → 403 ("contact the service operator to request access"), the rest → 401 with specific messages. Missing DESCOPE_PROJECT_ID becomes a startup hard-fail (a server-config bug, not a per-request 401).
• Tightened AuthResult so a reject always carries a failure (the illegal "rejected but failure=None" state is unrepresentable; ok = failure is None); AuthResult/AuthFailure exported for service authors who override the auth hook.

Author self-review

• No auth bypass: every reject path yields ok=False with a failure reason; _failure_response indexes an exhaustive table; no reject can return a 2xx (verified). The legacy boolean check_auth-override bridge still works.
• Error disclosure fails closed — only a positively-resolved non-trial tenant gets errors[]; messages name no company or product (framework-clean).
• Reviewed through the thermo-nuclear gate, which caught + drove a fix for the AuthResult illegal-state and a trial-tenant info-leak on the 500 path before this PR.

Related PRs

• vals-ai/benchmark-orchestrator#7 — the orchestrator-side half of Batch 2 (eval-result backfill + provider-key fail-fast). Same batch.

Type of Change

• Bug fix
• New feature
• Breaking change
• Refactor
• Docs / config

Testing

• Added/updated unit tests (226 passing; lint + typecheck clean)
• Manually tested

Checklist

• Self-reviewed the diff
• No debug/dead code left in
• Docs updated if needed

[assert-app]

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