fix(gateway): run channel-directory write off the event loop

atomic_json_write() calls os.fsync(), which blocks until the write
reaches stable storage. build_channel_directory() already offloads its
builders with asyncio.to_thread (#60794) but still called the persist
step directly on the loop, so the Discord heartbeat waited on a disk
flush.

gateway/channel_directory.py

@@ -206,7 +206,7 @@ async def build_channel_directory(adapters: Dict[Any, Any]) -> Dict[str, Any]:
     }
 
     try:
-        atomic_json_write(DIRECTORY_PATH, directory)
+        await asyncio.to_thread(atomic_json_write, DIRECTORY_PATH, directory)
     except Exception as e:
         logger.warning("Channel directory: failed to write: %s", e)