- Add payload detection and preparation helpers for compaction triggers and summary generation.
- Implement AES-GCM sealing and unsealing to store and restore opaque compaction capsules across turns.
- Support expanding compaction capsules into developer context for downstream turns.
- Provide summary text extraction across Gemini, Claude, and OpenAI response formats.
- Add builders for compaction non-stream JSON responses and SSE stream frames.
Closes: #5094
