Create a new LLM configuration for the authenticated user's space.

This endpoint:

• Requires API key or session authentication
• Validates that the secret_id belongs to the user's space
• Creates a new LLM configuration with the provided settings

Args: service: Injected LLM configuration service payload: Request containing url, display_name, model_name, and secret_id

Returns: Response with created LLM configuration

Raises: 400 Bad Request: If user doesn't have an associated space or secret is invalid