Update text_client_utils.py

[mzamini92]

This implementation adds a new get_available_models() method to the DebugClient class, which retrieves the list of available model configurations from the API and returns a list of their names. The send_message() method then calls this method and checks if the provided model_config_name is in the list of available models. If it's not, a ValueError is raised with an appropriate error message.

[AbdBarho]

we can probably get the model names once, no need to fetch every time.