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/language_model/src/language_model.rs

@@ -95,6 +95,12 @@ pub struct TokenUsage {
     pub cache_read_input_tokens: u32,
 }
 
+impl TokenUsage {
+    pub fn total_tokens(&self) -> u32 {
+        self.input_tokens + self.output_tokens
+    }
+}
+
 impl Add<TokenUsage> for TokenUsage {
     type Output = Self;