agent: Fix issues with usage display sometimes showing initially fetched usage (#33125)

Having `Thread::last_usage` as an override of the initially fetched
usage could cause the initial usage to be displayed when the current
thread is empty or in text threads. Fix is to just store last usage info
in `UserStore` and not have these overrides

Release Notes:

- Agent: Fixed request usage display to always include the most recently
known usage - there were some cases where it would show the initially
requested usage.

Cargo.lock

@@ -2821,6 +2821,7 @@ dependencies = [
  "cocoa 0.26.0",
  "collections",
  "credentials_provider",
+ "derive_more",
  "feature_flags",
  "fs",
  "futures 0.3.31",
@@ -2859,6 +2860,7 @@ dependencies = [
  "windows 0.61.1",
  "workspace-hack",
  "worktree",
+ "zed_llm_client",
 ]
 
 [[package]]
@@ -8159,6 +8161,7 @@ name = "inline_completion"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "client",
  "gpui",
  "language",
  "project",