Improve model name in metadata

[sergioburdisso]

When generating objects (Dialog, Persona, etc.) that contain metadata about the model used to generate them, we should also store the model name string now that we support multiple sources for models (e.g. "openai:gpt-4.1", "ollama:llama3:30B", "huggingface:user/model", etc.)

[sergioburdisso]

Implementation: have a global function "get_model_name()" or "get_model_metadata()" to return the desired output, and then refactor the whole code so that everywhere in the code only this function is used to get the name.