Update src/toon_format/decoder.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: jreakin

src/toon_format/decoder.py

@@ -329,6 +329,8 @@ def decode(input_str: str, options: Optional[DecodeOptions] = None) -> Union[Jso
 
     # If json_indent is specified, return JSON-formatted string
     if options.json_indent is not None:
+        if options.json_indent < 0:
+            raise ValueError(f"json_indent must be non-negative, got {options.json_indent}")
         return json.dumps(result, indent=options.json_indent, ensure_ascii=False)
 
     return result