[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_human_output.rs

@@ -222,7 +222,15 @@ impl EventProcessor for EventProcessorWithHumanOutput {
             EventMsg::BackgroundEvent(BackgroundEventEvent { message }) => {
                 ts_msg!(self, "{}", message.style(self.dimmed));
             }
-            EventMsg::StreamError(StreamErrorEvent { message, .. }) => {
+            EventMsg::StreamError(StreamErrorEvent {
+                message,
+                additional_details,
+                ..
+            }) => {
+                let message = match additional_details {
+                    Some(details) if !details.trim().is_empty() => format!("{message} ({details})"),
+                    _ => message,
+                };
                 ts_msg!(self, "{}", message.style(self.dimmed));
             }
             EventMsg::TaskStarted(_) => {

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(),
         }

codex-rs/exec/tests/event_processor_with_json_output.rs

@@ -583,6 +583,7 @@ fn stream_error_event_produces_error() {
         EventMsg::StreamError(codex_core::protocol::StreamErrorEvent {
             message: "retrying".to_string(),
             codex_error_info: Some(CodexErrorInfo::Other),
+            additional_details: None,
         }),
     ));
     assert_eq!(