feat(etl-uvicorn): carry failure_category through invoke and precheck responses

[CyMule]

Summary

• add optional failure_category to InvokeResponse and InvokePrecheckResponse, populated from a failure_category attribute on the raised error (same pattern as the existing status_code pickup), and thread it through the /precheck route
• lets plugins report the preflight failure_category contract (DTPL-525) through the standard precheck_func= wiring; today the response model silently drops the field, which forced the partitioner to strip and reinstall the SDK's precheck routes (Unstructured-IO/platform-plugins#1780)

Follow-up after release: revert the partitioner to precheck_func= wiring and delete its route-stripping in main.py (DTPL-694).

Testing

• PYTHONPATH=. uv run pytest test/api/test_api.py — 43 passed, including new coverage for a failing precheck carrying failure_category and a passing precheck leaving it null
• ruff check clean (the one ruff format complaint in api_generator.py predates this change)

[CyMule]

Superseded by #77.