hosted_rooms: add cross-process lock to prevent state.db corruption on concurrent gateway startup

[Sahilvishnaliya]

Fixes #102120

The hosted_room_worker starts on every profile gateway restart and calls
prune_disbanded_rooms() on the shared state.db. When multiple profiles
restart simultaneously (e.g. during 'hermes update' fleet restart), all
gateways hit the same SQLite database concurrently, causing corruption
('file is not a database').

This fix adds a cross-process advisory lock (fcntl on Unix, msvcrt on
Windows) that serializes access to the shared database during pruning.
The lock file is created alongside the database with a .hosted_rooms.lock
suffix.

Also adds a graceful fallback when neither locking mechanism is available
(no-op lock that at least doesn't crash).

---

Testing: All 44 hosted_rooms tests pass.

[alt-glitch]

This was generated by AI during triage.

Related: #102153 addresses the reported concurrent first-open/WAL setup mechanism for #102120. This PR locks pruning instead; reviewers should choose whether that separate scope is needed.

[Sahilvishnaliya]

🔴 CI failing — needs a fix before merge.

Failing check:

• Python lints / ruff + ty diff — failure

Other jobs (tests, other lints) are passing. Please check the ruff + ty diff job log — likely a ruff lint violation or ty type-check regression introduced in this diff — and push a fix.

[Sahilvishnaliya]

CI Python lints / ruff + ty diff cause confirmed in the lock prelude of gateway/hosted_rooms.py.

1. Unused import os (F401) — the new lock code never uses os.
2. msvcrt possibly unbound (ty) — it is only assigned inside except ImportError for fcntl, but later referenced as elif msvcrt:.
3. Dead _is_lock_contention_errno — defined, never called; that also makes import errno unused once the helper is removed.

Fix the top of the file to:

from __future__ import annotations

import contextlib
import hashlib
import json
import re
import sqlite3
import time
from contextlib import contextmanager
from pathlib import Path
from typing import Any, Iterator, Mapping, NoReturn

# Cross-process file locking: fcntl on Unix, msvcrt on Windows.
fcntl = None
msvcrt = None
try:
    import fcntl
except ImportError:
    try:
        import msvcrt
    except ImportError:
        pass

And delete _is_lock_contention_errno entirely (unused).

Do not nest import msvcrt only under the fcntl ImportError without first setting msvcrt = None — that is the ty unbound-name failure on Unix.