fix(orb-broker): validate App credentials only after enrollment is proven valid

[JSONbored]

Summary

• brokerOrbToken checked ORB_GITHUB_APP_ID/ORB_GITHUB_APP_PRIVATE_KEY before looking up the enrollment secret, so an unauthenticated caller with any bad/unknown secret could distinguish "broker misconfigured" (503) from "invalid secret" (401) via the response code alone — leaking the server's deployment-config state without ever proving a valid enrollment. Moves the credential check to immediately before the mint call (the only place it's actually needed — a cache hit never needs it), so an invalid/unknown secret always yields invalid_enrollment regardless of server configuration.
• Follow-up to fix(orb-broker): improve error handling for token exchange failures #2710, which merged before this ordering issue (flagged during that PR's own review) was addressed.

Scope

• The PR title follows type(scope): short summary Conventional Commit format, for example fix(api): restore profile access checks.
• This PR is focused and does not mix unrelated backend, UI, MCP, docs, dependency, and deploy changes.
• This follows CONTRIBUTING.md and does not reintroduce GitHub Pages, VitePress, site/, or CNAME.
• I linked an issue, or this is small enough that the summary explains why an issue is not needed. (No pre-existing issue; a single-function ordering fix with full test coverage, discovered via the review gate on fix(orb-broker): improve error handling for token exchange failures #2710.)

Validation

• git diff --check
• npm run typecheck
• npm run test:coverage locally (scoped to src/orb/broker.ts — 100% patch coverage on the changed lines/branches)
• New or changed behavior has unit/integration tests for new branches, fallback paths, and sanitizer boundaries

Safety

• No secrets, wallet details, hotkeys, coldkeys, user PATs, private keys, raw trust scores, private rankings, or private maintainer evidence are exposed.
• Public GitHub text stays sanitized, low-noise, and does not imply compensation guarantees or optimization tactics.
• Auth, cookie, CORS, GitHub App, Cloudflare, or session changes include negative-path tests. (Regression test asserts an invalid secret always returns invalid_enrollment, even with zero App credentials configured.)
• API/OpenAPI/MCP behavior is updated and tested where needed. (No public API contract change — same result shapes/status codes, only the internal check order changed.)
• UI changes use live API data or real empty/error/loading states, not production mock/demo fallbacks. (N/A — no UI changes.)
• Public docs/changelogs are updated where needed; changelogs are only edited for release-prep PRs. (N/A — internal broker ordering fix, no public-facing doc change.)

UI Evidence

N/A — backend-only change, no UI.

Notes

• The eligibility check (installation_not_eligible) still runs before the credential check, so a revoked/suspended install never leaks config state either.
• A still-fresh cached token is served without ever touching the credential check, since readCachedOrbToken only depends on TOKEN_ENCRYPTION_SECRET.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@86dde13). Learn more about missing BASE report.
⚠️ Report is 6 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2733   +/-   ##
=======================================
  Coverage        ?   96.02%           
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  Files           ?      244           
  Lines           ?    27300           
  Branches        ?     9919           
=======================================
  Hits            ?    26214           
  Misses          ?      460           
  Partials        ?      626           

Files with missing lines	Coverage Δ
src/orb/broker.ts	100.00% <100.00%> (ø)

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