gh-142916: fix mkdir TOCTOU race condition

[fatelei]

fix mkdir TOCTOU race condition

• Issue: pathlib.Path.mkdir() raises FileExistsError for non-existent path due to TOCTOU race condition #142916

[adriancaruana]

This PR doesn't actually fix the race condition; it makes the race window smaller but doesn't eliminate it. The retry adds extra syscalls in the error path with no clear benefit since it doesn't eliminate the race. The same failure mode can still occur:

• Retry mkdir() -> raises FileExistsError (another thread created it)
• Check is_dir() -> returns False (that thread deleted it again)
• Incorrectly raises FileExistsError for non-existent path