assistant2: Make Esc cancel current completion (#23076)

This PR makes it so pressing `Esc` in Assistant2 will cancel the current
completion.

Release Notes:

- N/A

crates/assistant2/src/thread.rs

@@ -502,6 +502,17 @@ impl Thread {
             };
         }
     }
+
+    /// Cancels the last pending completion, if there are any pending.
+    ///
+    /// Returns whether a completion was canceled.
+    pub fn cancel_last_completion(&mut self) -> bool {
+        if let Some(_last_completion) = self.pending_completions.pop() {
+            true
+        } else {
+            false
+        }
+    }
 }
 
 #[derive(Debug, Clone)]