[ci] Retry transient empty NAT responses

[antoniomtz]

Summary

• Retry a NAT functional request once, after two seconds, only when NAT returns the known empty-LLM 422 workflow_error.
• Emit a visible GitHub Actions warning when the targeted retry is used.
• Keep timeouts, other HTTP errors, malformed responses, and response-contract failures single-attempt.

Root cause

After #123 merged, Blueprint QA reached the search agent but the upstream model returned neither content nor tool calls. NAT surfaced that transient condition as HTTP 422. A rerun on the same commit passed all NAT checks and the UI suite.

Validation

• pytest tests/: 394 passed
• Ruff lint and format checks passed for backend, tests, and the NAT checker
• Pyright passed for the NAT checker and backend targets
• Blueprint QA passed all NAT functional checks and the UI suite with the retry implementation
• git diff --check passed

Follow-up to #123.