generateSpeech method
Generate speech from text using Google's native TTS
request - The TTS request configuration
Returns audio data and metadata or throws an LLMError
Implementation
@override
Future<GoogleTTSResponse> generateSpeech(GoogleTTSRequest request) async {
  try {
    final requestBody = request.toJson();
    // Use the appropriate TTS model if not specified
    final model = request.model ?? _config.model;
    final response = await _client.post(
      'models/$model:generateContent',
      data: requestBody,
    );
    return GoogleTTSResponse.fromApiResponse(
        response.data as Map<String, dynamic>);
  } catch (e) {
    throw GenericError('Google TTS generation failed: $e');
  }
}