Build SDK LLM class requires id for on-demand and base_id for on-demand-lora

Based on early feedback from users and internal testing, we've refactored the LLM class deployment logic in the Build SDK to make it easier to understand.

Key changes:

• The id parameter is now required when deployment_type is "on-demand"
• The base_id parameter is now required when deployment_type is "on-demand-lora"
• The deployment_display_name parameter is now optional and defaults to the filename where the LLM was instantiated

A new deployment will be created if a deployment with the same id does not exist. Otherwise, the existing deployment will be reused.