fix: nanobot onboard update config crash

when use onboard and choose N,
maybe sometimes will be crash and
config file will be invalid.

nanobot/config/loader.py

@@ -5,7 +5,6 @@ from pathlib import Path
 
 from nanobot.config.schema import Config
 
-
 # Global variable to store current config path (for multi-instance support)
 _current_config_path: Path | None = None
 
@@ -59,7 +58,7 @@ def save_config(config: Config, config_path: Path | None = None) -> None:
     path = config_path or get_config_path()
     path.parent.mkdir(parents=True, exist_ok=True)
 
-    data = config.model_dump(by_alias=True)
+    data = config.model_dump(mode="json", by_alias=True)
 
     with open(path, "w", encoding="utf-8") as f:
         json.dump(data, f, indent=2, ensure_ascii=False)