Reject HTTP error responses before partitioning

[CyMule]

Summary

Reject HTTP 4xx/5xx responses before classifying and partitioning their bodies. Previously a failed URL fetch could ingest the server's error page as if it were the requested document. Preserve successful response handling and timeout forwarding.

Compatibility

This intentionally changes failed-fetch behavior: callers that previously accepted an HTTP error page now receive ValueError with the HTTP status. It adds no document-size or format limit. Successful 200/206 responses retain their original bytes and MIME-detection inputs.

The existing metrics-test change serializes shared result-directory access across pytest-xdist workers to prevent one test deleting another's output.

Validation

Synced with current main on September 14, 2026.

• URL helper/status/timeout checks: 10 passed, covering 400/401/403/404/429/500/503 errors and successful 200/206 responses.
• Metrics suite with eight workers: 26 passed.
• Ruff and diff whitespace checks passed.
• Fresh full CI pending.