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/core/src/tools/handlers/extension_tools.rs

@@ -517,6 +517,7 @@ mod tests {
             transparent_background: None,
             failure: None,
             saved_path: None,
+            imagegen_request_id: None,
         });
         let expected_completed_item = ExtensionItem::ImageGeneration(ImageGenerationItem {
             id: "call-image".to_string(),
@@ -526,6 +527,7 @@ mod tests {
             transparent_background: Some(true),
             failure: None,
             saved_path: Some(expected_path.clone()),
+            imagegen_request_id: None,
         });
         codex_tools::TurnItemEmitter::emit_started(
             &emitter,