Add 'raw' option to --force-reasoning for native tokenizer prompt seeding

- Added 'raw' to valid force-reasoning options (chat, stop, inject, prompt, twopass, mock, raw)
- Implemented raw mode handler that:
  - Uses tokenizer.apply_chat_template() with add_generation_prompt=True
  - Seeds reasoning tag + commitment sentence
  - Uses two-pass generation: first captures reasoning, then gets final answer
  - Supports both streaming and non-streaming responses
  - Falls back gracefully if tokenizer not available

This enables using the model's native tokenizer for prompt seeding, bypassing
double-templating issues with chat APIs.