Remember max mode setting per-thread and add a user setting (#30042)

Supersedes: https://github.com/zed-industries/zed/pull/29936

Thanks for your contribution @imumesh18, but we had a slightly different
take on it :)

Release Notes:

- N/A

Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>

Cargo.lock

@@ -612,6 +612,7 @@ dependencies = [
  "serde_json_lenient",
  "settings",
  "workspace-hack",
+ "zed_llm_client",
 ]
 
 [[package]]