POST /api/session/:id/prompt silently drops unknown keys (e.g. model) instead of rejecting them

[shuv1337]

Summary

PromptInput.Prompt (packages/schema/src/prompt-input.ts:29-34) only accepts text, files, agents, skills. A body that also carries model (or any unknown key) is accepted with HTTP 200 and the extra key is silently discarded, so callers believe they selected a model when they did not. Combined with #1 (agent model ignored) this produced turns running on an unintended free model with no error anywhere.

Repro (integration-v2 @ 4668dfc)

POST /api/session/<id>/prompt
{"text":"Reply with exactly: ok","model":{"providerID":"anthropic","modelID":"claude-sonnet-5"}}
→ 200 {"data":{"id":"msg_…"}}

The turn runs on the catalog default model; the model key is never validated or applied. (By contrast, omitting text correctly yields 400 InvalidRequestError: Missing key at ["text"].)

Expected

Either reject unknown keys on the prompt body with a 400 (strict decode), or accept model on the prompt and apply it for that turn (matching POST /api/session which does accept model, and session.switchModel).

Notes

Model.Ref uses id, not modelID, which is a second easy footgun for API users — worth a schema-level rejection so the mismatch is visible.