Change Summary

POST /multi_search with union=true&conversation=true was sending empty data to the conversation model. CollectionManager::do_union() writes results as top-level hits, not a results[] array. The conversation post-processing only iterated response["results"], which doesn't exist in union responses, so the model always received [].

The fix adds a dedicated branch for the union case that reads documents from the top-level hits array and gathers vector fields from all collections involved in the union.

Test Plan

• Started a dummy model server (test/dummy_model_server.py) that logs the full request body received from Typesense
• Without fix: POST /multi_search?conversation=true&union=true with two searches against hnstories, the model received [] (empty data) even though the union search found hits. The response also contained "results": null
• With fix: The model receives the hits correctly for the same request

PR Checklist

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