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/scripts/update_sdk_artifacts.py

@@ -591,6 +591,7 @@ def generate_v2_all(schema_dir: Path, *, format_output: bool = True) -> None:
             cwd=sdk_root() if format_output else Path(td),
         )
     _preserve_inline_image_class_names(out_path)
+    _add_legacy_image_generation_thread_item_alias(out_path)
     _require_nullable_field(out_path, "ChatgptAccount", r"email: str \| None")
     _require_nullable_field(out_path, "McpResourceReadTarget", r"link_id: Annotated\[[\s\S]*?\]")
     _preserve_open_enum(out_path, "CodexErrorInfoValue", allow_empty=True)
@@ -618,6 +619,22 @@ def _preserve_inline_image_class_names(out_path: Path) -> None:
     out_path.write_text(source)
 
 
+def _add_legacy_image_generation_thread_item_alias(out_path: Path) -> None:
+    """Keep the inline image-generation class import available after splitting the wire type."""
+    source = out_path.read_text()
+    class_start = source.find("class ResultThreadItem(BaseModel):")
+    if class_start == -1:
+        raise RuntimeError("Generated SDK is missing ResultThreadItem")
+    if re.search(r"\bImageGenerationThreadItem\b", source):
+        raise RuntimeError("Generated SDK already defines ImageGenerationThreadItem")
+    class_end = source.find("\n\nclass ", class_start)
+    if class_end == -1:
+        class_end = len(source)
+
+    alias = "\n\nImageGenerationThreadItem = ResultThreadItem\n"
+    out_path.write_text(source[:class_end] + alias + source[class_end:])
+
+
 def _require_nullable_field(out_path: Path, class_name: str, field_pattern: str) -> None:
     """Preserve required-but-nullable fields that codegen gives a None default."""
     source = out_path.read_text()