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/thread-store/src/local/rollout_migration/legacy_event.rs

@@ -166,6 +166,7 @@ pub(super) fn completed_item(
                 transparent_background: event.transparent_background,
                 failure: event.failure.clone(),
                 saved_path: event.saved_path.clone(),
+                imagegen_request_id: None,
             })),
             None,
         )),

codex-rs/thread-store/src/local/rollout_migration_tests.rs

@@ -441,6 +441,7 @@ async fn migration_preserves_image_generation_failure_metadata() {
             resets_at: Some(1_786_150_800),
         }),
         saved_path: None,
+        imagegen_request_id: None,
     };
     let image_completion =
         RolloutItem::EventMsg(EventMsg::ImageGenerationEnd(ImageGenerationEndEvent {