When a stale lock file survives a gateway crash, `acquire_scoped_lock()`
may return `(False, existing_dict)` even after detecting and deleting
the stale lock (e.g. if unlink fails or a race condition occurs).
Previously, `_acquire_platform_lock()` called
`_set_fatal_error(..., retryable=False)`, which permanently killed the
platform — the reconnect watcher never retries a non-retryable fatal
error.
Change to `retryable=True` so the platform enters the "retrying"
state and the reconnect watcher can attempt acquisition again after the
standard backoff delay.
Fixes#54167
