Repro: no ParticipantConnected for participants already in room at connect

[MaxHeimbrock]

Summary

Deterministic reproduction of the field reports where a user joins a room concurrently with a remote participant (typically an agent being dispatched) and never realizes that participant is there — no ParticipantConnected, and consequently no app-side track subscription/rendering.

This PR is intentionally red. The test documents the current defect; the fix follows on this branch.

Mechanism

Whether a concurrently connecting participant lands in the local participant's connect snapshot or in the event stream is a server-side race (did it join before or after the local JoinResponse was generated). If it lands in the snapshot:

• the Rust SDK creates it silently from join_response.other_participants (livekit/src/room/mod.rs:744 — only handle_participant_update ever dispatches ParticipantConnected, and only for participants not already known),
• the FFI packs it into the ConnectCallback payload (build_initial_states) without events,
• Unity adds it to RemoteParticipants via CreateRemoteParticipantWithTracks (Room.cs:596) without raising room-level ParticipantConnected or TrackPublished.

So even a handler wired before Connect() never fires. The data is all there — only the events are missing. The ReadyForRoomEvent handshake is not at fault: the buffered-delta path it protects was verified watertight at all three layers.

Test design

Three participants make the race deterministic and the failure fast (~3s, no timeout burn):

1. An "agent" fully connects first → guaranteed into the subscriber's snapshot.
2. The subscriber wires ParticipantConnected before Connect(), then connects.
3. A late joiner connects after → its ParticipantConnected (regular delta path) serves as an in-order control: room events are delivered in order, so once the control event fired, the agent's event would already have been dispatched if it were ever coming.

Current failure output:

ParticipantConnected never fired for 'snapshot-agent', which was already in the
room when the subscriber connected (snapshot participant). It IS present in
RemoteParticipants, so only the event is missing. Received: [snapshot-late-joiner]

Test plan

• Scripts~/run_unity.sh test -m PlayMode -f SnapshotParticipantEventsTests against livekit-server --dev — fails on the repro assertion, control assertions pass
• Goes green with the upcoming fix (synthesizing the events for snapshot participants in Room.OnConnect)

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