fix: suppress citation SSE events for unresolved (hallucinated) citation IDs

[octo-patch]

Fixes #2224

Problem

When streaming RAG responses, the LLM sometimes generates citation IDs that do not correspond to any retrieved document chunk (i.e., hallucinated citations). In SSEFormatter.yield_citation_event, the previous condition

if not citation_data.get("is_new") or "payload" not in citation_data:
    citation_data["payload"] = None

treated a missing "payload" key (new citation, source not found) the same as a repeated citation (where nulling the payload is intentional). As a result, the server emitted streaming events like:

{"id":"c910e2e","object":"citation","is_new":true,"span":{"start":411,"end":420},"payload":null}

Clients receiving is_new=true with payload=null cannot distinguish between a genuinely new citation (where they expect full source data) and a hallucinated one.

Solution

Split the condition into two explicit branches:

1. Repeated citation (is_new=false): set payload=null as before — the client already has the payload from the first occurrence.
2. New citation, source not found (is_new=true, no "payload" key): silently suppress the event. The citation ID was not in the retrieved chunks, so there is nothing meaningful to send. The citation also does not appear in the final final_answer event (that path already filters on citation_payloads), keeping streaming and final-answer behaviour consistent.
3. New citation, source found (is_new=true, "payload" key present): unchanged — emits with full payload.

Testing

• Repeated citations still receive payload=null.
• New citations with a valid source still emit full payload.
• Hallucinated citation IDs (not in search_results_collector) no longer produce a streaming event, consistent with the final-answer behaviour.