`_lock_for` caches one threading.Lock per (dir, file, pid) in the module-global
`_WRITER_LOCKS`, but nothing evicted those entries: `delete_run_journal` rmtree'd
the on-disk `_run_journal/{sid}/` directory yet left the cached lock objects
behind, so a long-lived gateway leaked one entry per deleted run forever.
Delete now evicts every cached lock whose parent directory matches the removed
session (pid-independent) under `_WRITER_LOCKS_GUARD`, leaving unrelated
sessions' locks intact.
Refs #4633, #2097.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
