Track image generation request IDs in analytics (#40714)

## What changed

- Read `x-codex-imagegen-request-id` from image generation and edit responses.
- Propagate the ID to `codex_image_generation_event` analytics.
- Keep the ID in process only, excluding it from extension item serialization, generated types, app-server wire data, and rollout history.

## Testing

- Cover response-header extraction and missing-header behavior in the images client.
- Verify end-to-end analytics emission and the unchanged image-generation item wire shape.

GitOrigin-RevId: 9b8e09b7d655c0c2faa84f0eee99aabae583668d

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

@@ -973,6 +973,7 @@ impl From<CoreTurnItem> for ThreadItem {
                     transparent_background: None,
                     failure: None,
                     saved_path: image.saved_path,
+                    imagegen_request_id: None,
                 })
             }
             CoreTurnItem::EnteredReviewMode(review) => ThreadItem::EnteredReviewMode {