Handle pytest.exit from workers

[gaoflow]

Summary

• propagate pytest.exit() details from workers to the controller process
• stop treating a worker that exits via pytest.exit() as a crashed worker with a pending crash item
• re-raise pytest's Exit in the controller so the requested return code is preserved
• add coverage for return codes 0 and 1

Closes #1239

Tests

• uv run --python 3.11 --with '.[testing]' pytest testing/acceptance_test.py::TestNodeFailure::test_pytest_exit_is_not_reported_as_internal_error -q
• uv run --python 3.11 --with '.[testing]' pytest testing -q
• uv run --python 3.11 --with '.[testing]' --with ruff ruff check src/xdist/dsession.py src/xdist/remote.py testing/acceptance_test.py
• uv run --python 3.11 --with '.[testing]' --with mypy --with setproctitle mypy src/xdist/dsession.py src/xdist/remote.py testing/acceptance_test.py
• git diff --check