agent: Add token count in the thread view (#28037)

This PR adds the token count to the active thread view. It doesn't
behaves quite like Assistant 1 where it updates as you type, though; it
updates after you submit the message.

<img
src="https://github.com/user-attachments/assets/82d2a180-554a-43ee-b776-3743359b609b"
width="700" />

---

Release Notes:

- agent: Add token count in the thread view

---------

Co-authored-by: Agus Zubiaga <hi@aguz.me>

crates/assistant_context_editor/src/context_editor.rs

@@ -3703,6 +3703,18 @@ pub fn humanize_token_count(count: usize) -> String {
                 format!("{}.{}k", thousands, hundreds)
             }
         }
+        1_000_000..=9_999_999 => {
+            let millions = count / 1_000_000;
+            let hundred_thousands = (count % 1_000_000 + 50_000) / 100_000;
+            if hundred_thousands == 0 {
+                format!("{}M", millions)
+            } else if hundred_thousands == 10 {
+                format!("{}M", millions + 1)
+            } else {
+                format!("{}.{}M", millions, hundred_thousands)
+            }
+        }
+        10_000_000.. => format!("{}M", (count + 500_000) / 1_000_000),
         _ => format!("{}k", (count + 500) / 1000),
     }
 }