Support provider-side compaction in the language model clients (#59145)

Adds provider-side context compaction support to the Anthropic and
OpenAI API clients. Client plumbing only; not wired into the UI.

Release Notes:

- N/A
33a54ce423Agus Zubiaga committed on 6/16/2026, 1:06:20 PM· committed by GitHubparent4c0717f
28 files changedLine totals unavailable