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/app-server-protocol/src/protocol/thread_history.rs

@@ -862,6 +862,7 @@ impl ThreadHistoryBuilder {
             status: String::new(),
             revised_prompt: None,
             result: String::new(),
+            transparent_background: None,
             saved_path: None,
         });
         self.upsert_item_in_current_turn(item);
@@ -873,6 +874,7 @@ impl ThreadHistoryBuilder {
             status: payload.status.clone(),
             revised_prompt: payload.revised_prompt.clone(),
             result: payload.result.clone(),
+            transparent_background: payload.transparent_background,
             saved_path: payload.saved_path.clone(),
         });
         self.upsert_item_in_current_turn(item);
@@ -2076,6 +2078,7 @@ mod tests {
                         status: "completed".to_string(),
                         revised_prompt: Some("A blue square".to_string()),
                         result: "cG5n".to_string(),
+                        transparent_background: Some(true),
                         saved_path: Some(saved_path.clone()),
                     },
                 )),
@@ -2106,6 +2109,7 @@ mod tests {
                 status: "completed".to_string(),
                 revised_prompt: Some("A blue square".to_string()),
                 result: "cG5n".to_string(),
+                transparent_background: Some(true),
                 saved_path: Some(saved_path),
             })]
         );
@@ -2368,6 +2372,7 @@ mod tests {
                 status: "completed".into(),
                 revised_prompt: Some("final prompt".into()),
                 result: "Zm9v".into(),
+                transparent_background: Some(true),
                 saved_path: Some(test_path_buf("/tmp/ig_123.png").abs()),
             })),
             RolloutItem::EventMsg(EventMsg::TurnComplete(TurnCompleteEvent {
@@ -2407,6 +2412,7 @@ mod tests {
                         status: "completed".into(),
                         revised_prompt: Some("final prompt".into()),
                         result: "Zm9v".into(),
+                        transparent_background: Some(true),
                         saved_path: Some(test_path_buf("/tmp/ig_123.png").abs()),
                     }),
                 ],

codex-rs/app-server-protocol/src/protocol/v2/item.rs

@@ -928,6 +928,7 @@ impl From<CoreTurnItem> for ThreadItem {
                     status: image.status,
                     revised_prompt: image.revised_prompt,
                     result: image.result,
+                    transparent_background: None,
                     saved_path: image.saved_path,
                 })
             }