fix(windows): preserve mkstemp errors through cleanup

[knewstimek]

When _wopen fails, cleanup can overwrite errno before cbm_mkstemp checks whether to retry or returns the error. Save the error immediately and restore it after cleanup; preserve random-generation and path-conversion failures as well.

Follow-up to #2282.

Validation on Windows x64: the retained-file and non-ASCII-directory tests pass. A focused harness makes cleanup overwrite errno: the original code returns EIO instead of ENOENT for a missing parent directory, while this change preserves ENOENT.

[github-actions]

Thanks for opening this — it has been seen, and it is queued.

This note is automated, but it is not a brush-off: it exists so you know where your PR stands instead of having to guess from silence.

Current review status: working through a backlog. 0.9.1-rc.1 is out, so the release freeze that held reviews is over — but it left a large queue of open pull requests behind it, and we are reading through them oldest-first. The background is in discussion #1144.

What that means for this PR, concretely:

• It will not be closed for inactivity. No stale bot touches pull requests here.
• It may still sit a while before a human reads it. That is on us, not on you.
• Older PRs are read first, so a recent one is not being skipped — it is behind a queue.

Things that will genuinely speed it up whenever review does happen:

• Keep it rebased on main — the tree is moving quickly right now, and a conflicting branch cannot be reviewed as the diff you intended.
• Get CI green, or say which failures you believe are pre-existing.
• Keep the change to one claim. Bundled features and refactors get split before they get merged, which costs you a round trip.
• Every commit needs a sign-off (git commit -s) — CI enforces DCO.

If this fixes a bug, a reproduction we can run is worth more than a description of the symptom.

Thanks for contributing, and sorry in advance for the wait.

[DeusData]

Merged. Thank you, @knewstimek, for following up on the #2282 review so quickly! errno from _wopen is now saved right away and restored after the cleanup path, so a failed mkstemp reports the real cause instead of whatever the cleanup left behind. It's small, and exactly the kind of care that makes Windows failures diagnosable. It went in on a fully green matrix, and nothing merged since touches compat.c.