Refusal Message and Category Improvements

Refusal handling has been refactored to better distinguish between model-level safety measures and topic-specific restrictions:

• frontier_llm and reasoning_extraction are now recognized as known refusal categories with dedicated handling.
• Cybersecurity refusals now surface an exemption link: https://claude.com/form/cyber-use-case
• Refusal messages now explicitly call out which model triggered the block (using a readable model name via Mp() rather than a raw model ID).
• The stop_details.category type is now an open string schema to forward new categories before schema updates ship.