Switches buildRequest to emit anthReq.MultiSystem instead of anthReq.System
whenever a system message is present. Upstream's MarshalJSON prefers
MultiSystem when non-empty, so the wire format is unchanged for requests
without cache_control. This refactor is a prerequisite for attaching
cache_control markers to system parts in the next commit.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
