Set the native-app agent orchestration model.

Sets the orchestration model to auto or a lowercase model name (^(auto|[a-z0-9][a-z0-9._-]{0,63})$; input is trimmed and lowercased). A named model that is not available on the account is persisted as auto with status fallback (still a 200 — read setting.status and setting.message; status pending means the saved model is not applied to the agent yet (the admin card shows "Not applied yet"), so save again to retry). A model the engine rejects is a 400 carrying its error. Snowflake mode only (404 elsewhere); requires the connection:manage capability (admin or owner; an API key needs that scope), own org only. Runs under the caller's own Snowflake session: 409 when that session has no usable warehouse. Audit-logged.