copilot: Fix cache control error (#56632)

#56472 broke Copilot chat

> Failed to connect to API: 400 Bad Request {"message":"cache_control:
Extra inputs are not permitted"}

This PR makes it so that we still use the legacy caching approach for
Copilot

Release Notes:

- N/A
956dbea79aBennet Bo Fenner committed on 5/13/2026, 1:34:47 PM· committed by GitHubparent6aaea4d
5 files changedLine totals unavailable