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/items/src/image_generation.rs

@@ -38,4 +38,12 @@ pub struct ImageGenerationItem {
     #[serde(default, skip_serializing_if = "Option::is_none")]
     #[ts(optional)]
     pub saved_path: Option<AbsolutePathBuf>,
+    /// Exact nested ImageGen request ID retained only for in-process analytics.
+    ///
+    /// This is deliberately excluded from the extension/app-server wire shape
+    /// and rollout persistence because it is not a client-facing item field.
+    #[serde(skip)]
+    #[schemars(skip)]
+    #[ts(skip)]
+    pub imagegen_request_id: Option<String>,
 }