agent: Respect compaction boundaries in request windows (#58241)

Agent request construction now honors persisted compaction markers by
rebuilding the model-visible window from the latest boundary, retaining
recent user context for summary compactions, and treating
provider-native compactions as suffix-only until native replay lands.

Release Notes:

- N/A

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>