Use read-only access methods for read-only entity operations (#31254)

This PR replaces some `update()` calls with either `read()` or
`read_with()` when the `update()` call performed read-only operations on
the entity.

Many more likely exist, will follow-up with more PRs.

Release Notes:

- N/A

crates/agent/src/context_picker/completion_provider.rs

@@ -1213,7 +1213,7 @@ mod tests {
             assert_eq!(worktrees.len(), 1);
             worktrees.pop().unwrap()
         });
-        let worktree_id = worktree.update(cx, |worktree, _| worktree.id());
+        let worktree_id = worktree.read_with(cx, |worktree, _| worktree.id());
 
         let mut cx = VisualTestContext::from_window(*window.deref(), cx);