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/image_generation.rs

@@ -15,6 +15,9 @@ pub struct ImageGenerationItem {
     pub status: String,
     pub revised_prompt: Option<String>,
     pub result: String,
+    #[serde(default)]
+    #[ts(optional)]
+    pub transparent_background: Option<bool>,
     #[serde(default, skip_serializing_if = "Option::is_none")]
     #[ts(optional)]
     pub saved_path: Option<AbsolutePathBuf>,