Use the same prompt as agent thread summary for text threads (#35669)

This was causing text thread summarization to be counted as a usage of 1
prompt

Release Notes:

- Fixed bug with agent text threads (not chat threads) counting
summarization as a usage of 1 prompt.

Co-authored-by: Oleksiy <oleksiy@zed.dev>
86957a5614Michael Sloan committed on 8/5/2025, 9:47:17 PM· committed by GitHubparent4269941
3 files changedLine totals unavailable