Fix xdist collisions for captured_info artifacts and preserve CI debug logs

[stationeros]

What does this PR do?

Fixes #45561.

This PR fixes a race in the test failure reporting flow when the suite is run with pytest-xdist.

Right now every worker writes debug output to the same captured_info.txt file. In parallel runs that means workers can overwrite each other’s output, and in some cases one worker can remove the file another worker was still relying on. The end result is flaky or incomplete failure logs, which makes CI failures much harder to debug.

The fix keeps the current behavior for non-xdist runs, but switches to worker-specific files when PYTEST_XDIST_WORKER is set. So instead of multiple workers all writing to the same file, each one writes its own captured_info_<worker>.txt.

Along with that, this PR also updates the surrounding plumbing so the debug info still shows up where people expect it:

• CI log collection now prints all matching captured_info*.txt files instead of assuming there is only one.
• notification artifact handling aggregates worker-specific captured info files back into the existing reporting flow.
• stale captured_info*.txt files are cleaned up before a new collection starts so repeated runs do not pick up leftovers from earlier failures.

I also added tests covering:

• worker-specific path selection under xdist
• legacy single-file behavior outside xdist
• cleanup of stale captured info files
• aggregation of worker-specific artifacts in notification handling

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

Who can review?

This touches test infrastructure / CI behavior, so @ydshieh seems like the most relevant reviewer.