Room: Add leave method #75

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

async def leave(self) -> None

Example

await room.leave()

Before opening a PR

• Write unit tests
• Run mypy matrix/
• Run pytest tests/
• Run black .