Store generated image items as attachment references (#54398)

## What changed

- Upload generated images to the attachment store and expose `fileId` in image-generation items when available, with inline `result` fallback on upload or decoding failure.
- Preserve file references through legacy events, persisted rollouts, thread history, and task output summaries. Keep model-facing image preparation separate.
- Add attachment request tags and mark generated-image uploads with `usage=image_gen`, preserving the thread's ephemeral setting.
- Update public schemas and SDK types to accept either image representation, retain the Python `ImageGenerationThreadItem` alias, and prefer `fileId` when reading records containing both fields.

## Testing

Add coverage for file-backed generation and rollout persistence, legacy history replay, inline and transitional serialization, task output visibility, and Python SDK alias compatibility.

GitOrigin-RevId: f7a2bb5707e2f277348a547a988bc33c8e96d891

sdk/python/tests/test_artifact_workflow_and_binaries.py

@@ -556,6 +556,30 @@ def test_generated_inline_image_class_names_remain_stable() -> None:
     )
 
 
+def test_generated_image_generation_thread_item_alias_remains_available() -> None:
+    """Existing SDK callers can still construct and validate inline image-generation items."""
+    from openai_codex.generated.v2_all import (
+        ImageGenerationThreadItem,
+        ResultThreadItem,
+    )
+
+    item = ImageGenerationThreadItem.model_validate(
+        {
+            "failure": None,
+            "id": "image_1",
+            "result": "aW1hZ2U=",
+            "revisedPrompt": None,
+            "savedPath": None,
+            "status": "completed",
+            "transparentBackground": None,
+            "type": "imageGeneration",
+        }
+    )
+
+    assert ImageGenerationThreadItem is ResultThreadItem
+    assert isinstance(item, ResultThreadItem)
+
+
 def test_runtime_package_template_has_no_checked_in_binaries() -> None:
     runtime_root = ROOT.parent / "python-runtime" / "src" / "codex_cli_bin"
     assert sorted(