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

codex-rs/app-server-protocol/src/protocol/v2/item.rs

@@ -15,6 +15,7 @@ use crate::protocol::item_builders::convert_patch_changes;
 use crate::protocol::item_builders::review_output_text;
 use codex_experimental_api_macros::ExperimentalApi;
 use codex_extension_items::ExtensionItem;
+pub use codex_extension_items::image_generation::GeneratedImageRef;
 pub use codex_extension_items::image_generation::ImageGenerationFailure;
 pub use codex_extension_items::image_generation::ImageGenerationItem;
 pub use codex_extension_items::sleep::SleepItem;
@@ -1009,7 +1010,9 @@ impl From<CoreTurnItem> for ThreadItem {
                     id: image.id,
                     status: image.status,
                     revised_prompt: image.revised_prompt,
-                    result: image.result,
+                    image: GeneratedImageRef::Inline {
+                        result: image.result,
+                    },
                     transparent_background: None,
                     failure: None,
                     saved_path: image.saved_path,