feat: Make ParalleToolCalls nullable

[HavenDV]

It should be nullable and null by default because API will return error for simple chat requests:

{
  "error": {
    "message": "Invalid value for 'parallel_tool_calls': 'parallel_tool_calls' is only allowed when 'tools' are specified.",
    "type": "invalid_request_error",
    "param": "parallel_tool_calls",
    "code": null
  }
}