fix: MBFunctionCall stored JSON strings instead of raw values (double-encoding)

[alubbock]

Summary

• capture_function_args_and_kwargs called self.to_json(v) and stored the resulting JSON string in bm_data['args']/bm_data['kwargs']. When output_result later serialised bm_data, these strings were serialised again, turning e.g. {'k': 1} into the escaped string "{\"k\": 1}" in the output.
• Fixed to call self.to_json(v) only as an encodability check, then store the raw value v — matching how MBReturnValue handles return values
• Adds test_functioncall_args_not_double_encoded to verify args/kwargs round-trip as their native Python types