feat: Add optional maxRoomSize to prevent room flooding

[Emelie-Dev]

This PR defends against room flooding by introducing a configurable MAX_ROOM_SIZE environmental boundary.

Changes
Room Capacity: The RoomManager now respects an optional maxRoomSize setting.
Server Handlers: If a join_room message results in a rejection because the room is at capacity, the client is sent an error frame featuring a ROOM_FULL code.
Environment: Documented MAX_ROOM_SIZE within README.md and added a default limit of 500 to .env.example.
Testing: Additional unit tests have been committed to ensure full-capacity rejections and standard server error framing function smoothly.
Acceptance Criteria met
RoomManager constructor gracefully accepts { maxRoomSize }.
join() rejects when at capacity ({ ok: false, reason: 'ROOM_FULL' }).
join() processes accurately when below capacity.
Server transmits a WebSocket error frame to any client whose join was rebuffed.
.env.example and README.md accurately track this parameter.
Tested under-capacity and over-capacity edge cases.

closes #173

[levibliz]

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