assistant2: Wire up error messages (#21426)

This PR wires up the error messages for Assistant 2 so that they are
shown to the user:

<img width="1138" alt="Screenshot 2024-12-02 at 4 28 02 PM"
src="https://github.com/user-attachments/assets/d8a5b9bd-0cef-4304-b561-b2edadbc70ef">
<img width="1138" alt="Screenshot 2024-12-02 at 4 29 09 PM"
src="https://github.com/user-attachments/assets/0dd70841-0d5a-4de6-bebe-82c563246b65">
<img width="1138" alt="Screenshot 2024-12-02 at 4 32 49 PM"
src="https://github.com/user-attachments/assets/a8838866-fad1-43a9-8935-490dc1936016">

@danilo-leal I kept the existing UX from Assistant 1, as I didn't see
any errors in the design prototype, but we can revisit if another
approach would work better.

Release Notes:

- N/A

crates/language_model/src/language_model.rs

@@ -55,7 +55,7 @@ pub enum LanguageModelCompletionEvent {
     StartMessage { message_id: String },
 }
 
-#[derive(Debug, PartialEq, Clone, Serialize, Deserialize)]
+#[derive(Debug, PartialEq, Clone, Copy, Serialize, Deserialize)]
 #[serde(rename_all = "snake_case")]
 pub enum StopReason {
     EndTurn,