Follow-up: Remove model_class_provider from prompt files

Problem

In !3485 (merged), a fix was introduced which allows the system to use the correct provider from the model's definition in models.yml, enabling proper fallback behavior when model-specific prompts don't exist. In the previous MR, the use model_class_provider was only specified for OpenAI models.

Proposal

1. Extend model_class_provider to all models - Currently only OpenAI models (GPT-5, GPT-5-mini, GPT-5-Codex) have model_class_provider added in models.yml. This needs to be extended to all other models (Anthropic, Mistral, etc.).
2. Remove model_class_provider from prompt files - Once all models have model_class_provider defined in models.yml, the redundant model_class_provider declarations can be cleaned up from all prompt definition files.