Avoid spawning external agent process when AI is disabled at startup (#39649)

Closes #39645 

Release Notes:

- Fixed external agent servers sometimes being spawned when Zed started
even when AI was disabled.
f6f11eb544Cole Miller committed on 10/7/2025, 1:01:17 AM· committed by GitHubparentc1e9171
2 files changedLine totals unavailable