Allow fallback to base, with overridden model_class_provider

Problem

Currently, prompt files contain a model_class_provider field that couples prompts to specific model providers. When on self-hosted environments, falling back to base prompts with model_class_provider: anthropic causes hard failures

Current State

• model_class_provider is defined in prompt files (e.g., model_class_provider: anthropic in base prompts)
• Prompt selection and model provider are unnecessarily coupled

Proposed Solution

1. Allow fallback to base, with overridden model_class_provider - Done in !3485 (merged)

Follow -up: Will be done in #1529

1. Rely on models.yml to determine model_class_provider for ALL models.
2. Remove model_class_provider from ALL models.

This will fully decouple prompts from models, allowing:

• Base prompts to be truly model-agnostic
• Proper fallback behavior without compliance violations
• Self-hosted AIGW to work correctly with model selection