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/ext/image-generation/src/backend.rs

@@ -82,7 +82,7 @@ impl CodexImagesBackend {
         &self,
         request: ImageGenerationRequest,
         turn_id: &str,
-    ) -> Result<ImageResponse, ImageBackendError> {
+    ) -> Result<(ImageResponse, Option<String>), ImageBackendError> {
         self.client()
             .await?
             .generate(
@@ -98,7 +98,7 @@ impl CodexImagesBackend {
         &self,
         request: ImageEditRequest,
         turn_id: &str,
-    ) -> Result<ImageResponse, ImageBackendError> {
+    ) -> Result<(ImageResponse, Option<String>), ImageBackendError> {
         self.client()
             .await?
             .edit(