[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/tui/src/chatwidget/tests.rs

@@ -3223,11 +3223,13 @@ async fn stream_error_updates_status_indicator() {
     let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(None).await;
     chat.bottom_pane.set_task_running(true);
     let msg = "Reconnecting... 2/5";
+    let details = "Idle timeout waiting for SSE";
     chat.handle_codex_event(Event {
         id: "sub-1".into(),
         msg: EventMsg::StreamError(StreamErrorEvent {
             message: msg.to_string(),
             codex_error_info: Some(CodexErrorInfo::Other),
+            additional_details: Some(details.to_string()),
         }),
     });
 
@@ -3241,6 +3243,7 @@ async fn stream_error_updates_status_indicator() {
         .status_widget()
         .expect("status indicator should be visible");
     assert_eq!(status.header(), msg);
+    assert_eq!(status.details(), Some(details));
 }
 
 #[tokio::test]
@@ -3277,6 +3280,7 @@ async fn stream_recovery_restores_previous_status_header() {
         msg: EventMsg::StreamError(StreamErrorEvent {
             message: "Reconnecting... 1/5".to_string(),
             codex_error_info: Some(CodexErrorInfo::Other),
+            additional_details: None,
         }),
     });
     drain_insert_history(&mut rx);