Review follow-up: if os.fdopen() raised before taking ownership of the
descriptor returned by os.open(), the cleanup handler unlinked the temp
file but leaked the fd. Close it explicitly on that path, mirroring the
credential-writer cleanup from #62837.
Strengthen the tests so the old writer could not pass them: an os.open
spy asserts O_CREAT | O_EXCL and an explicit 0o600 mode (the final-mode
check alone was also satisfied by the post-write chmod), and a forced
fdopen-failure test asserts the raw fd is closed and no temp file is
left behind.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
c3531cfac2