Fix citations

[DidierRLopes]

What went wrong

• No stable IDs: The UI renders citation markers from each citation.id. Early on, citations were sent without a usable id, so markers didn’t render. A refresh backfilled ids and injected markers from history, which is why they appeared only after reload.
• Wrong timing: When citations were emitted before any message chunks, the UI appended markers to an empty aiMessageBuffer — so citations
  showed at the top.

What the UI expects

• A live AI message must exist when markers arrive. useStreamResponse creates it on the first copilotMessageChunk (or citation) and treats the last AI message as the “stream target”.
• Citations append to the current aiMessageBuffer at the moment the copilotCitationCollection event is processed.
• The markers reference citation.id exactly; ids must be strings.

What fixed it

• Guarantee string ids on each citation before streaming.
• Stream LLM chunks first, then send copilotCitationCollection so markers land at the end of the final message.
• Yield typed SSE objects and serialize once at EventSourceResponse to match the UI event schema precisely.

Resulting event order

• copilotMessageChunk " " → creates the AI message and starts streaming
• copilotMessageChunk "…tokens…"
• …
• copilotCitationCollection [{ id: "…" }, …] → markers appear at the end
• done

[piiq]

Closing in favor of #76