Handle standalone image generation failures as terminal items (#27920)

## Why

Standalone image generation emitted a started item but no terminal item
when the backend failed. Clients could leave the operation unresolved or
render it as successful.

## What changed

- Emit a terminal image-generation item with `status: "failed"` when
generation or editing fails.
- Skip image persistence for failed terminal items.
- Render failed image generation distinctly in TUI history.
- Preserve the status when handling live and replayed terminal items.

## Looks for TUI, App-Side change needed 

<img width="867" height="89" alt="image"
src="https://github.com/user-attachments/assets/9e32342f-a982-411e-8498-456639fc468a"
/>

## Validation

- `just test -p codex-image-generation-extension`
- App-server image-generation tests
- Core stream-event tests
- TUI image-generation lifecycle and snapshot tests
- Scoped Clippy and formatting

codex-rs/tui/src/chatwidget/replay.rs

@@ -151,11 +151,12 @@ impl ChatWidget {
             }
             ThreadItem::ImageGeneration {
                 id,
+                status,
                 revised_prompt,
                 saved_path,
                 ..
             } => {
-                self.on_image_generation_end(id, revised_prompt, saved_path);
+                self.on_image_generation_end(id, status, revised_prompt, saved_path);
             }
             ThreadItem::EnteredReviewMode { review, .. } => {
                 if from_replay {

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__failed_image_generation_call_history_snapshot.snap

@@ -0,0 +1,6 @@
+---
+source: tui/src/chatwidget/tests/exec_flow.rs
+expression: "lines_to_single_string(&cells[0])"
+---
+✗ Image generation failed
+  └ A tiny blue square

codex-rs/tui/src/chatwidget/tests/exec_flow.rs

@@ -864,6 +864,7 @@ async fn image_generation_call_adds_history_cell() {
     handle_image_generation_end(
         &mut chat,
         "call-image-generation",
+        "completed",
         Some("A tiny blue square".into()),
         Some(test_path_buf("/tmp/ig-1.png").abs()),
     );
@@ -876,6 +877,21 @@ async fn image_generation_call_adds_history_cell() {
     let combined =
         lines_to_single_string(&cells[0]).replace(&platform_file_url, "file:///tmp/ig-1.png");
     assert_chatwidget_snapshot!("image_generation_call_history_snapshot", combined);
+
+    handle_image_generation_end(
+        &mut chat,
+        "call-image-generation-failed",
+        "failed",
+        Some("A tiny blue square".into()),
+        /*saved_path*/ None,
+    );
+
+    let cells = drain_insert_history(&mut rx);
+    assert_eq!(cells.len(), 1, "expected a single failure history cell");
+    assert_chatwidget_snapshot!(
+        "failed_image_generation_call_history_snapshot",
+        lines_to_single_string(&cells[0])
+    );
 }
 
 #[tokio::test]

codex-rs/tui/src/chatwidget/tests/helpers.rs

@@ -694,6 +694,7 @@ pub(super) fn handle_view_image_tool_call(
 pub(super) fn handle_image_generation_end(
     chat: &mut ChatWidget,
     call_id: impl Into<String>,
+    status: impl Into<String>,
     revised_prompt: Option<String>,
     saved_path: Option<AbsolutePathBuf>,
 ) {
@@ -704,7 +705,7 @@ pub(super) fn handle_image_generation_end(
             completed_at_ms: 0,
             item: AppServerThreadItem::ImageGeneration {
                 id: call_id.into(),
-                status: "completed".to_string(),
+                status: status.into(),
                 revised_prompt,
                 result: String::new(),
                 saved_path,

codex-rs/tui/src/chatwidget/tool_lifecycle.rs

@@ -29,12 +29,14 @@ impl ChatWidget {
     pub(super) fn on_image_generation_end(
         &mut self,
         call_id: String,
+        status: String,
         revised_prompt: Option<String>,
         saved_path: Option<AbsolutePathBuf>,
     ) {
         self.flush_answer_stream_with_separator();
         self.add_to_history(history_cell::new_image_generation_call(
             call_id,
+            &status,
             revised_prompt,
             saved_path,
         ));

codex-rs/tui/src/history_cell/patches.rs

@@ -73,15 +73,17 @@ pub(crate) fn new_view_image_tool_call(path: AbsolutePathBuf, cwd: &Path) -> Pla
 
 pub(crate) fn new_image_generation_call(
     call_id: String,
+    status: &str,
     revised_prompt: Option<String>,
     saved_path: Option<AbsolutePathBuf>,
 ) -> PlainHistoryCell {
-    let detail = revised_prompt.unwrap_or_else(|| call_id.clone());
-
-    let mut lines: Vec<Line<'static>> = vec![
-        vec!["• ".dim(), "Generated Image:".bold()].into(),
-        vec!["  └ ".dim(), detail.dim()].into(),
-    ];
+    let detail = revised_prompt.unwrap_or(call_id);
+    let heading = if status == "failed" {
+        vec!["✗ ".red().bold(), "Image generation failed".bold()].into()
+    } else {
+        vec!["• ".dim(), "Generated Image:".bold()].into()
+    };
+    let mut lines: Vec<Line<'static>> = vec![heading, vec!["  └ ".dim(), detail.dim()].into()];
     if let Some(saved_path) = saved_path {
         let saved_path = Url::from_file_path(saved_path.as_path())
             .map(|url| url.to_string())

codex-rs/tui/src/history_cell/tests.rs

@@ -474,6 +474,7 @@ fn image_generation_call_renders_saved_path() {
     );
     let cell = new_image_generation_call(
         "call-image-generation".to_string(),
+        "completed",
         Some("A tiny blue square".to_string()),
         Some(saved_path),
     );