fix: retry transient failures in the weekly online fixture refresh

[kevincostner17]

Summary

The scheduled Refresh online fixtures workflow has failed three weeks running (08-31, 09-07, 09-14). The 09-14 run died on:

http.client.RemoteDisconnected: Remote end closed connection without response

_download made a single urlopen call with no retry. RemoteDisconnected (and IncompleteRead) subclass HTTPException/OSError, not URLError, so the per-dataset except didn't catch them. One dropped connection from archive.ics.uci.edu aborted the entire run, and every remaining dataset was skipped. Caught failures (e.g. the 08-31 Connection reset by peer) also turned the job red, because any single failure exited 1.

Changes

• scripts/fixture_download.py (new): download() retries OSError/HTTPException, HTTP 429 and 5xx with exponential backoff and jitter (4 attempts). Permanent 4xx errors are raised immediately; once retries run out it raises TransientFetchError.
• scripts/fetch_online_fixtures.py:
  • Continues past a failed dataset.
  • Classifies each failure as either network (retries exhausted) or dataset (parse error, empty after parse, hash mismatch, permanent HTTP error).
  • New --max-failures N sets how many network failures are tolerated (default 0). Dataset failures always fail the run.
  • A network failure keeps the dataset's existing manifest entry.
  • Emits ::warning:: annotations under GitHub Actions.
• tests/expectations.py: the live-fetch helper reuses download().
• .github/workflows/fetch-fixtures.yml:
  • Runs with --max-failures 2.
  • The offline verify step now uses if: always() and -m "not online"; the live fetch already ran in the previous step.
  • Adds a 30-minute timeout.
  • Opens or updates a nightly-failure issue on scheduled failures, like the other nightly lanes.

Tests

New tests/test_fixture_download.py (11 tests, no network):

• retries on RemoteDisconnected and connection reset, with backoff
• gives up after N attempts
• retries 429/503; doesn't retry 404
• main() keeps going after a network failure and respects the budget
• parse, permanent-HTTP and empty-after-parse failures are never tolerated
• warning annotations under GitHub Actions

Results:

• pytest tests/test_fixture_download.py: 11 passed
• pytest tests/test_online_datasets.py -m "not online": 170 passed
• ruff check on the changed files: clean

[coderabbitai]

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