Preserve ImageUserInput in the Python SDK (#45796)

Keep the existing public class name for URL-based image input when regenerating the SDK. Map `UrlUserInput` to `ImageUserInput` during artifact generation and update the generated model and `UserInput` union accordingly.

Extend the class-name stability test to assert that `ImageUserInput` remains importable under its expected name.

GitOrigin-RevId: 6cbbd555e2e07904cc5fbc279e1e61ff089ac0a9

sdk/python/scripts/update_sdk_artifacts.py

@@ -587,6 +587,7 @@ def _preserve_inline_image_class_names(out_path: Path) -> None:
     """Keep the public class names used before ImageReference was introduced."""
     source = out_path.read_text()
     stable_names = {
+        "UrlUserInput": "ImageUserInput",
         "ImageUrlContentItem": "InputImageContentItem",
         "ImageUrlFunctionCallOutputContentItem": "InputImageFunctionCallOutputContentItem",
     }