Assert the cache metadata of responses built by the engine

Problem/Motivation

A response assembled from shapes, nested cards, paragraphs, menus and service calls collects cache metadata from a dozen places, and losing a single tag produces the worst kind of bug: a response that is correct when tested and stale in production until a full cache flush. The current resources handle cacheability inconsistently — an earlier review of caching found exactly that — and the new engine multiplies the number of places it can be dropped.

Proposed resolution

• A test helper that renders a response and asserts its complete cache metadata: tags, contexts and max-age.
• Per shape, assert that the entities and configuration it reads appear in the tags — a card carries the tags of the referenced entity, a menu block the tags of the menu, a service call whatever the service declared.
• Assert the contexts of a listing actually cover its query parameters, which is where a wrong page gets served today.
• Assert invalidation end to end: edit the referenced entity, request again, get the new value.

Remaining tasks

Everything.