Propose Formation subgroups without creating anything

Asks the model to propose a subgroup hypothesis for this Audience from a free-text request (e.g. "split by purchase frequency"). Nothing is persisted; pass the returned hypothesis (edited if you like) to POST /audiences/{audienceId}/formations to build it, or re-post with priorHypothesis set to the previous proposal to request a rework. Any Audience viewer may call this. Optional JSON bodies are limited to 64 KiB; userInput is a string of at most 2000 trimmed characters. Invalid supplied priorHypothesis values return 400. Responses, including refusals, are private and non-cacheable. Disconnecting the request or cancelling the stream aborts model generation. Returned proposals satisfy the creation input contract. Send Accept: application/json to receive one JSON document. Accept quality weights, wildcards, and media parameters are respected; when neither JSON nor NDJSON is acceptable the route returns 406 before generation. With no Accept preference the response is a newline-delimited JSON stream (application/x-ndjson) of { type, data } events in the order meta, status, subgroups, done, with failures reported in-band as { type: "error", error } followed by done.