[chore] add additional_details to StreamErrorEvent + wire through (#8307)

### What

Builds on #8293.

Add `additional_details`, which contains the upstream error message, to
relevant structures used to pass along retryable `StreamError`s.

Uses the new TUI status indicator's `details` field (shows under the
status header) to display the `additional_details` error to the user on
retryable `Reconnecting...` errors. This adds clarity for users for
retryable errors.

Will make corresponding change to VSCode extension to show
`additional_details` as expandable from the `Reconnecting...` cell.

Examples:
<img width="1012" height="326" alt="image"
src="https://github.com/user-attachments/assets/f35e7e6a-8f5e-4a2f-a764-358101776996"
/>

<img width="1526" height="358" alt="image"
src="https://github.com/user-attachments/assets/0029cbc0-f062-4233-8650-cc216c7808f0"
/>

codex-rs/exec/src/event_processor_with_jsonl_output.rs

@@ -145,9 +145,15 @@ impl EventProcessorWithJsonOutput {
                 };
                 vec![ThreadEvent::ItemCompleted(ItemCompletedEvent { item })]
             }
-            EventMsg::StreamError(ev) => vec![ThreadEvent::Error(ThreadErrorEvent {
-                message: ev.message.clone(),
-            })],
+            EventMsg::StreamError(ev) => {
+                let message = match &ev.additional_details {
+                    Some(details) if !details.trim().is_empty() => {
+                        format!("{} ({})", ev.message, details)
+                    }
+                    _ => ev.message.clone(),
+                };
+                vec![ThreadEvent::Error(ThreadErrorEvent { message })]
+            }
             EventMsg::PlanUpdate(ev) => self.handle_plan_update(ev),
             _ => Vec::new(),
         }