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/analytics/src/analytics_client_tests.rs

@@ -4786,6 +4786,7 @@ async fn turn_event_counts_completed_tool_items() {
             status: "completed".to_string(),
             revised_prompt: None,
             result: "ok".to_string(),
+            transparent_background: None,
             saved_path: None,
         }),
     ];