fcntl is not available on Windows. This adds msvcrt.locking as a
fallback for cross-process advisory locking on Windows.
msvcrt.locking is not reentrant within the same thread, unlike fcntl.flock.
This matters because resolve_codex_runtime_credentials holds the lock and
then calls _save_codex_tokens, which tries to acquire it again. Without
reentrancy tracking, this deadlocks on Windows after a 15-second timeout.
Uses threading.local() to track lock depth per thread, allowing nested
acquisitions to pass through without re-acquiring the underlying lock.
Also handles msvcrt-specific requirements: file must be opened in r+ mode
(not a+), must have at least 1 byte of content, and the file pointer must
be at position 0 before locking.
48e65631f6