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

sdk/python/src/openai_codex/generated/v2_all.py

@@ -5558,6 +5558,13 @@ class FileIdThreadItem(BaseModel):
         ),
     ] = None
     failure: ImageGenerationFailure | None = None
+    generation_id: Annotated[
+        str | None,
+        Field(
+            alias="generationId",
+            description="Image generation identifier for the generated image, when provided by the image backend.",
+        ),
+    ] = None
     id: str
     input_images: Annotated[
         list[ImageGenerationInputImage] | None,
@@ -5593,6 +5600,13 @@ class ResultThreadItem(BaseModel):
         ),
     ] = None
     failure: ImageGenerationFailure | None = None
+    generation_id: Annotated[
+        str | None,
+        Field(
+            alias="generationId",
+            description="Image generation identifier for the generated image, when provided by the image backend.",
+        ),
+    ] = None
     id: str
     input_images: Annotated[
         list[ImageGenerationInputImage] | None,