[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/core/src/codex.rs

@@ -1422,12 +1422,14 @@ impl Session {
         message: impl Into<String>,
         codex_error: CodexErr,
     ) {
+        let additional_details = codex_error.to_string();
         let codex_error_info = CodexErrorInfo::ResponseStreamDisconnected {
             http_status_code: codex_error.http_status_code_value(),
         };
         let event = EventMsg::StreamError(StreamErrorEvent {
             message: message.into(),
             codex_error_info: Some(codex_error_info),
+            additional_details: Some(additional_details),
         });
         self.send_event(turn_context, event).await;
     }