Optional ReadonlyattachmentsMethod state stored with a reused prefix.
ReadonlycachesRestored, a reused prefix (composed as the graph's caches), or fresh.
Optional ReadonlycontinuationA resumed generation: caches cover its prompt and generated tokens.
Optional ReadonlyretainReleases the backing of reused state; run it after disposing caches.
ReadonlyreusedPrompt tokens caches already cover; 0 is a miss.
A request's state, owned by the caller.