Unbind a tile from its query model

Unbind a query-presentation tile from its query model on a draft, keeping the tile’s query. No request body.

The query-model binding (query.model_extension_id) is server-owned on the whole-document PATCH surface — a supplied value there is ignored and the tile keeps its stored binding. This draft-scoped route is the way to change it, addressing one tile by its stable record key. The binding shifts on every draft (each draft re-clones its query models), so work against a draft you have already read.