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/thread_history.rs

@@ -859,6 +859,7 @@ impl ThreadHistoryBuilder {
             transparent_background: None,
             failure: None,
             saved_path: None,
+            imagegen_request_id: None,
         });
         self.upsert_item_in_current_turn(item);
     }
@@ -872,6 +873,7 @@ impl ThreadHistoryBuilder {
             transparent_background: payload.transparent_background,
             failure: payload.failure.clone(),
             saved_path: payload.saved_path.clone(),
+            imagegen_request_id: None,
         });
         self.upsert_item_in_current_turn(item);
     }
@@ -2083,6 +2085,7 @@ mod tests {
                         transparent_background: Some(true),
                         failure: None,
                         saved_path: Some(saved_path.clone()),
+                        imagegen_request_id: None,
                     },
                 )),
                 started_at_ms: Some(0),
@@ -2115,6 +2118,7 @@ mod tests {
                 transparent_background: Some(true),
                 failure: None,
                 saved_path: Some(saved_path),
+                imagegen_request_id: None,
             })]
         );
     }
@@ -2432,6 +2436,7 @@ mod tests {
                             },
                         ),
                         saved_path: Some(test_path_buf("/tmp/ig_123.png").abs()),
+                        imagegen_request_id: None,
                     }),
                 ],
             }

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 {