Preserve image transparency metadata in app-server items (#37178)

## What changed

- Add `transparentBackground` to image-generation items and legacy completion events.
- Map Images API backgrounds to `true` for transparent, `false` for opaque, and `null` for automatic or unavailable values.
- Preserve the metadata in live notifications and persisted or resumed thread history while continuing to deserialize older items that omit it.

## Testing

- Cover transparent, opaque, and automatic backgrounds, including image edits and thread read/resume flows.
- Verify the stable JSON shape and optional TypeScript field.

GitOrigin-RevId: 08527f37f23431df45fab1ed34eb82b2139b49d7

codex-rs/ext/items/src/tests.rs

@@ -1,5 +1,6 @@
 use pretty_assertions::assert_eq;
 use serde_json::json;
+use ts_rs::TS;
 
 use super::ExtensionItem;
 use super::image_generation::ImageGenerationItem;
@@ -13,6 +14,7 @@ fn completed_image_generation_item() -> ExtensionItem {
         status: "completed".to_string(),
         revised_prompt: Some("A blue square".to_string()),
         result: "cG5n".to_string(),
+        transparent_background: None,
         saved_path: None,
     })
 }
@@ -30,12 +32,58 @@ fn image_generation_item_preserves_stable_wire_shape() {
             "status": "completed",
             "revisedPrompt": "A blue square",
             "result": "cG5n",
+            "transparentBackground": null,
         })
     );
     assert_eq!(
         serde_json::from_value::<ExtensionItem>(value).expect("deserialize extension item"),
         item
     );
+    assert_eq!(
+        serde_json::from_value::<ExtensionItem>(json!({
+            "kind": "image_gen.generation",
+            "id": "image-1",
+            "status": "completed",
+            "revisedPrompt": "A blue square",
+            "result": "cG5n",
+        }))
+        .expect("deserialize legacy image-generation item without transparency metadata"),
+        item
+    );
+}
+
+#[test]
+fn image_generation_item_preserves_authoritative_transparency() {
+    let ExtensionItem::ImageGeneration(mut image) = completed_image_generation_item() else {
+        panic!("expected image-generation item");
+    };
+    image.transparent_background = Some(true);
+    let item = ExtensionItem::ImageGeneration(image);
+    let value = serde_json::to_value(&item).expect("serialize extension item");
+
+    assert_eq!(
+        value,
+        json!({
+            "kind": "image_gen.generation",
+            "id": "image-1",
+            "status": "completed",
+            "revisedPrompt": "A blue square",
+            "result": "cG5n",
+            "transparentBackground": true,
+        })
+    );
+    assert_eq!(
+        serde_json::from_value::<ExtensionItem>(value).expect("deserialize extension item"),
+        item
+    );
+}
+
+#[test]
+fn image_generation_transparency_is_optional_in_typescript() {
+    assert!(
+        ImageGenerationItem::inline().contains("transparentBackground?: boolean"),
+        "image-generation transparency must remain optional for existing TypeScript clients"
+    );
 }
 
 #[test]