streaming.WriteCompaction packages a 'compaction' event with the
kept/older turn counts, summary tokens and provider-reported
window/used tokens so the client can hint 'context optimized' to the
user without parsing the stream body.
streamChat runs Compact before BuildMessages and writes the event
right after start, ensuring the client sees it before any chunk is
emitted. Add a runner test that exercises limitRAGContext to keep
the system prompt + RAG block under the configured window.