acp: Support model selection for ACP agents (#38652)

It requires the agent to implement the (still unstable) model selection
API. Will allow us to test it out before stabilizing.

Release Notes:

- N/A
4e6e424fd7Ben Brandt committed on 9/22/2025, 3:07:40 PM· committed by GitHubparentdccbb47
13 files changedLine totals unavailable