add default value to the parameters

[feng-1985]

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In the tools.yaml, have config the default value of limit, but using the toolbox, it not parsed. (toolbox v0.25.0, toolbox-langchai [0.5.7])

course_tools = toolbox.load_toolset('course_information_tools')
course_tools[5].name
Out[11]: 'get_course_by_date'
course_tools[5].args
Out[10]: 
{'date': {'description': '\n输入日期。\r\nFormat: YYYY-MM-DD.\r\nIf not provided by the user, using the current system time by default.\r\n',
  'title': 'Date',
  'type': 'string'},
 'limit': {'anyOf': [{'type': 'integer'}, {'type': 'null'}],
  'default': None,
  'description': '输入返回结果数量。',
  'title': 'Limit'}}```


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