Fix `NotImplementedError` crash when using xdist schedulers without `mark_test_pending`

Handle gracefully when pytest-xdist schedulers (like `LoadScopeScheduling`) don't implement `mark_test_pending` by catching `NotImplementedError` and failing the test with a clear message instead of crashing the test run. When a test crashes and cannot be rescheduled: - Set report outcome to "failed" with informative error message - Explain which scheduler lacks rescheduling support - Show remaining rerun count that couldn't be used - Continue test execution instead of internal crash Fixes #247