98edba3683
Place user text before volatile turn metadata in outgoing user-message content arrays so provider prefix caches can continue matching the stable user-input prefix across date, model-route, and working-set changes. Also adds wire-level coverage proving tail-positioned turn metadata serializes after user text while preserving turn-meta deduplication. Harvested from PR #2517 by @HUQIANTAO Co-authored-by: HUQIANTAO <58421104+HUQIANTAO@users.noreply.github.com>