Fix: Report worker process exit codes in error messages

[0xkems]

Summary

• When a worker process terminates with a non-zero exit code (e.g., via native code calling exit() directly), the exit code is now included in the error message
• This helps diagnose crashes that bypass Python's exception handling mechanism
• Fixes If a worker exits with a non-zero status code pytest-xdist doesn't report failure #1278

Background

The original issue reported that when using Polars with timeout protection that directly terminates processes via native code, pytest-xdist doesn't report the failure. This is because such terminations bypass Python's exception handling, and previously the error message would only say "Not properly terminated".

Changes

• Added _get_worker_exit_code() method to WorkerController that safely retrieves the worker process exit code via the execnet gateway
• Modified process_from_remote() to check the exit code when the channel closes unexpectedly
• If a non-zero exit code is found, it's included in the error message (e.g., "Worker exited with non-zero exit code: 42")

Test plan

• Added new test test_exit_code_reported_in_error that verifies exit codes are reported
• Verified existing crash tests still pass
• Verified all test_workermanage.py tests pass

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