Preserve raw response usage metadata (#41980)

## What changed

- Include the complete upstream `response.usage` object in
  `ResponseUsageMetadata.metadata` while retaining the existing `amount` field.
- Expose the metadata through `rawResponse/completed` notifications and update
  the generated JSON and TypeScript schemas.

## Testing

- Cover SSE, WebSocket, turn, and compaction completion paths.

GitOrigin-RevId: 6d79b1b4c92ac11abf43ec98f380b4868288a701

codex-rs/app-server-protocol/schema/json/v2/RawResponseCompletedNotification.json

@@ -9,7 +9,8 @@
             "string",
             "null"
           ]
-        }
+        },
+        "metadata": true
       },
       "type": "object"
     },