Interaction dispatcher caches step pages, showing a stale step after the run advances

Problem/Motivation

The public interaction dispatcher (InteractionController::step()) renders whichever step the run is parked on at request time: a message, a payment page, a status. It returns those render arrays with no cache metadata, so the dynamic page cache (and the anonymous page cache) store the page keyed by its URL. When the run advances to the next step, the same dispatcher URL now resolves to a different step, but the cache replays the earlier render, so a visitor sees a stale step (for example "Awaiting confirmation" after a booking has already been confirmed) until caches are cleared.

The payment interaction hid this for its own page by stamping a zero max-age on its render, but message and status steps do not, so they go stale.

Proposed resolution

Stamp a zero max-age centrally on every array the dispatcher returns, so any parked step's page always reflects the live run state and is never served stale. A redirect (an interaction's off-site hand-off) manages its own cacheability.

Covered by a kernel test asserting the dispatched step response carries a zero max-age.

Remaining tasks

• Review.