Enrich event handlers with Room and improve Message API

[PenguinBoi12]

Previously, Matrix event handlers received a raw MatrixRoom object from nio, giving no access to our abstractions like send(). This PR passes our own Room wrapper instead, making the full bot API available directly inside event handlers.

Room:

• Event handlers now receive a Room instance instead of MatrixRoom, enabling calls to send(), invite_user(), etc. directly from handlers
• Added fetch_event to retrieve a raw nio Event by id from the homeserver
• Added fetch_message to retrieve a fully constructed Message by event id

Message:

• Message is now built with an Event object instead of just an event_id, giving access to all event data.
• Added fetch_reactions which returns a list[Reaction].

[chrisdedman]

Nice work. LGTM