Room: Add leave method

The bot currently has no way to leave a room at runtime. This is a common need, a bot may join a room temporarily, or be invited to one it should decline after inspecting. `Room.leave()` fills this gap and follows the exact same existing patterns.

### Proposed API
```python
async def leave(self) -> None
```

### Example
```python
await room.leave()
```

### Before opening a PR
- Write unit tests
- Run mypy matrix/
- Run pytest tests/
- Run black .