fix(claude): restore cloaked prompt-cache ownership and native shape

Cloaked Responses/Chat/Gemini→Claude traffic was skipping ensureCacheControl
once the first-user cloak marker existed, freezing multi-turn cache hits.
Always ensure section-independent system and rolling-message breakpoints for
non-native callers, keep confirmed Claude Code placement intact, and leave
tools unstamped when a usable system prompt already covers the prefix.

Align the wire shape with the Claude Code 2.1.220/2.1.221/2.1.227 cache-control
constructor: the default is {"type":"ephemeral"} with no ttl. A 1h body ttl is
applied only after placement, only for OAuth credentials, and only onto blocks
that already carry cache_control, so it stays strictly paired with
extended-cache-ttl-2025-04-11. Explicit caller-owned 5m choices survive.

Match the native rolling selector (skip thinking-like assistant tails; final
system string special case), fall back to a tools breakpoint when system is
absent or blank, and leave empty/whitespace string systems unconverted so they
cannot double-stamp tools plus a useless system host.