fix: dispose track publication handles on participant disconnect

[LautaroPetaccio]

Why

When the room processes a participantDisconnected event, it deletes the participant from the remoteParticipants map and emits an event — but never touches the participant's trackPublications. Each TrackPublication wraps an FfiHandle that maps to a native resource. Dropping the JS reference without calling dispose() means the native side never frees the handle. With N participants each publishing M tracks, every disconnect leaks N×M handles.

How

In the participantDisconnected event handler, before emitting the event, loop through participant.trackPublications, call ffiHandle.dispose() on each publication, and clear the map.

Test coverage

• cleans up track publications when a remote participant disconnects — publishes a track, disconnects the publisher, asserts trackPublications is empty on the observer side
• cleans up resources when multiple participants disconnect simultaneously — 4 participants each with a track, all disconnect at once

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@livekit/rtc-node	Patch

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[lukasIO]

for disconnecting participants we first receive a trackUnpublished event, which would arguably be the more obvious place to perform this cleanup.
Agree though, that the cleanup needs to happen at some point.

We could wrap it in queueMicrotask in the trackUnpublished handler.

[LautaroPetaccio]

The trackUnpublished handler currently doesn't dispose the FfiHandles either, it only removes the publication from the Map. So this would mean adding new disposal logic there, not moving it. I went with participantDisconnected as a single cleanup sweep that catches everything regardless of whether trackUnpublished fired for each track. Happy to move it if you feel strongly though.