Retry transient Docling readiness connections

[szweibel]

Summary

• use HTTPX built-in connection retries for remote Docling readiness
• align readiness with the processing client three-attempt connection boundary
• preserve the five-second per-attempt timeout and sanitized failure output

Evidence

• production single-attempt readiness was intermittent (5/10 connection failures)
• the same production boundary with two HTTPX connection retries passed 10/10
• 533 backend tests, frontend lint/typecheck/build, Ruff, and diff checks pass locally