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/src/request_processors/thread_resume_redaction.rs

@@ -1,3 +1,5 @@
+#[cfg(test)]
+use codex_app_server_protocol::GeneratedImageRef;
 use codex_app_server_protocol::McpToolCallResult;
 use codex_app_server_protocol::ThreadItem;
 use codex_app_server_protocol::Turn;
@@ -108,7 +110,9 @@ mod tests {
                 id: "ig-1".to_string(),
                 status: "completed".to_string(),
                 revised_prompt: Some("revised".to_string()),
-                result: "base64-result".to_string(),
+                image: GeneratedImageRef::Inline {
+                    result: "base64-result".to_string(),
+                },
                 transparent_background: None,
                 failure: None,
                 saved_path: Some(test_path_buf("/tmp/ig-1.png").abs()),