ci(unit-tests): Python 3.14 coverage guard intermittently finds an xdist worker with no data file #1423

Describe the bug
The Python 3.14 unit-tests job intermittently fails at Report coverage even though every test passes. The #1387 guard finds one xdist worker with no .coverage.* file.

Of the 30 most recent unit-tests.yml runs (02:34Z–13:52Z on 2026-10-08), all 3 hits came after 12:55Z:

• 37780418214 (main, the fix(ipv6): CALIPSO make() writes a corrupt length on overflow and raises raw errors for a bad pad #1409 merge): failed at Report coverage.
• 37780829236 (main, ec2ba13a4), attempt 1: 4 xdist workers, missing data: ['2458']; 3230 passed. The rerun passed.
• 37786185089 (fix(hip): three BitField namespaces leave reserved bits unnamed, so they rebuild as zero #1419): missing data: ['2593']; 3234 passed.

None of the 23 runs before 12:55Z hit it. That points at something that landed with #1409–#1412 (a test that kills, os._exits or replaces an xdist worker), but this is an inference, not yet shown.

Expected behavior
Every worker saves its coverage data, so the guard only fires when a worker really dies.