Expose and persist image generation IDs (#57552)

## What changed

Expose the backend-provided `generationId` on image-generation items and preserve it through legacy completion events, thread history reconstruction, and rollout migration. Default missing IDs to `None` for older history.

Update the generated JSON schemas, TypeScript types, and Python models to include the nullable field.

## Testing

Extend coverage for ID serialization round trips, completed-item notifications, persisted and resumed history, and legacy rollout migration.

GitOrigin-RevId: a7f06faa7ac6833417b257a535d2901c03b79135

codex-rs/app-server-protocol/schema/json/v2/ThreadMetadataUpdateResponse.json

@@ -2190,6 +2190,14 @@
               ],
               "default": null
             },
+            "generationId": {
+              "default": null,
+              "description": "Image generation identifier for the generated image, when provided by the image backend.",
+              "type": [
+                "string",
+                "null"
+              ]
+            },
             "id": {
               "type": "string"
             },