add single idempotence test for normpath (#61497)
AI Acknowledgement: I used GitHub Copilot (GPT Codex 5.3) for coding
assistance. I review and take responsibility for output.
This PR adds one single idempotence assertion to normpath, verifying
proper behavior and guarding against the potential corner case in which
normalizing an already normalized path changes its output. The function
should already be robust, but this test guards against the unlikely
event that a downstream change subtly changes behavior of normpath.
