[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/tui2/src/chatwidget.rs

@@ -959,11 +959,11 @@ impl ChatWidget {
         }
     }
 
-    fn on_stream_error(&mut self, message: String) {
+    fn on_stream_error(&mut self, message: String, additional_details: Option<String>) {
         if self.retry_status_header.is_none() {
             self.retry_status_header = Some(self.current_status_header.clone());
         }
-        self.set_status_header(message);
+        self.set_status(message, additional_details);
     }
 
     /// Periodic tick to commit at most one queued line to history with a small delay,
@@ -1905,9 +1905,11 @@ impl ChatWidget {
             }
             EventMsg::UndoStarted(ev) => self.on_undo_started(ev),
             EventMsg::UndoCompleted(ev) => self.on_undo_completed(ev),
-            EventMsg::StreamError(StreamErrorEvent { message, .. }) => {
-                self.on_stream_error(message)
-            }
+            EventMsg::StreamError(StreamErrorEvent {
+                message,
+                additional_details,
+                ..
+            }) => self.on_stream_error(message, additional_details),
             EventMsg::UserMessage(ev) => {
                 if from_replay {
                     self.on_user_message_event(ev);