Automatically retry in more situations (#34473)

In #33275 I was very conservative about when to retry when there are
errors in language completions in the Agent panel.

Now we retry in more scenarios (e.g. HTTP 5xx and 4xx errors that aren't
in the specific list of ones that we handle differently, such as 429s),
and also we show a notification if the thread halts for any reason.

<img width="441" height="68" alt="Screenshot 2025-07-15 at 12 51 30 PM"
src="https://github.com/user-attachments/assets/433775d0-a8b2-403d-9427-1e296d164980"
/>
<img width="482" height="322" alt="Screenshot 2025-07-15 at 12 44 15 PM"
src="https://github.com/user-attachments/assets/5a508224-0fe0-4d34-9768-25d95873eab8"
/>


Release Notes:

- Automatic retry for more Agent errors
- Whenever the Agent stops, play a sound (if configured) and show a
notification (if configured) if the Zed window was in the background.

crates/eval/src/example.rs

@@ -221,9 +221,6 @@ impl ExampleContext {
                 ThreadEvent::ShowError(thread_error) => {
                     tx.try_send(Err(anyhow!(thread_error.clone()))).ok();
                 }
-                ThreadEvent::RetriesFailed { .. } => {
-                    // Ignore retries failed events
-                }
                 ThreadEvent::Stopped(reason) => match reason {
                     Ok(StopReason::EndTurn) => {
                         tx.close_channel();