createOpenAIProvider function
Create an OpenAI provider with default settings
Implementation
OpenAIProvider createOpenAIProvider({
  required String apiKey,
  String model = ProviderDefaults.openaiDefaultModel,
  String baseUrl = ProviderDefaults.openaiBaseUrl,
  double? temperature,
  int? maxTokens,
  String? systemPrompt,
}) {
  final config = OpenAIConfig(
    apiKey: apiKey,
    model: model,
    baseUrl: baseUrl,
    temperature: temperature,
    maxTokens: maxTokens,
    systemPrompt: systemPrompt,
  );
  return OpenAIProvider(config);
}