test(candidate-execution): bound draining settlement retries

[drewstone]

Summary

• prove non-draining settlement errors, including 409 auth failures, are attempted once
• prove repeated candidate_grant_draining retries stop at the caller deadline
• prove the final draining error is preserved
• retain the existing execution-plus-settlement error precedence regression

Verification

• pnpm exec biome check --write tests/candidate-execution-model-port.test.ts
• pnpm exec vitest run tests/candidate-execution-model-port.test.ts --reporter=dot (41 passed)
• git merge-tree --write-tree origin/main HEAD (clean)

This is test-only coverage for the settlement retry behavior already shipped in Runtime 0.133.7. It does not require a version bump or release.

[tangletools]

✅ Auto-approved drewstone PR — bd1a5856

This PR was opened by the trusted drewstone account.
The full PR reviewer audit still runs separately and will publish findings if it detects issues.

This approval is provisional. It rests on the audit running. If the audit cannot run — for example the CLI bridge rejects it — this approval is dismissed rather than left standing, so an unrun check never reads as a passing one.

_{tangletools · auto-approval · reason: drewstone_author · 2026-08-13T20:09:28Z}