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

@@ -718,6 +718,7 @@ pub(super) fn handle_image_generation_end(
                 status: status.into(),
                 revised_prompt,
                 result: String::new(),
+                transparent_background: None,
                 saved_path,
             }),
         }),