fix(playground): key chat messages by sender and timestamp

[leepokai]

Problem

The chat list throws a React key collision during normal conversation:

Encountered two children with the same key, `1785829362590`.
Keys should be unique so that components maintain their identity across updates.
Non-unique keys may cause children to be duplicated and/or omitted.

The key is a millisecond timestamp, so this is not purely cosmetic — React warns that colliding keys can drop or duplicate rendered children, meaning chat lines can visibly disappear or repeat.

Root cause

MessageList keys each row by item.time alone:

return <MessageItem data={item} key={item.time} />;

item.time is text_ts, stamped with int(time.time() * 1000) when the transcript is sent, and item.userId is the sender's stream_id. The addChatItem reducer scopes both its discard and its replace branches to a single userId:

const LastFinalIndex = state.chatItems.findLastIndex(
  (el) => el.userId === userId && el.isFinal
);

So two transcripts from different senders that share a millisecond are both pushed, and the list renders two children with the same key.

Two transcripts from the same sender can never collide — the reducer either discards the later one (time <= LastFinalItem.time) or replaces the pending non-final item.

Speech-to-speech graphs make the cross-sender collision routine rather than rare: the final input transcription is emitted as the first output transcript delta arrives, and the two are sent with different stream_ids.

Fix

Key by sender and timestamp together, which is unique by the argument above.

Verification

Reproduced by replaying the reducer with a user transcript (stream_id from session metadata) and an agent transcript (stream_id 100) sharing one millisecond — two items, one key. With the composite key both rows are unique, and the same-sender case still collapses to a single item, so no row is lost.

Confirmed against a running voice_assistant_realtime graph: the warning fired twice during ordinary conversation before the change and does not appear after it.