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/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,
                 })
             }