添加更多模型定价数据

[CryptoSentinelAI]

当前 PRICING 字典只覆盖了 OpenAI 和 DeepSeek。需要补充：

• Gemini 2.5 系列
• Qwen 系列
• Mistral 系列
• Llama 4 系列

参考各平台官网定价页面。

Good first issue — 只需要更新 ai_llm/tokenizer.py 中的 PRICING 字典和 MODEL_ENCODING 映射。

[CryptoSentinelAI]

Thanks for flagging this! A few notes for anyone picking this up:

• Pricing data lives in ai_llm/tokenizer.py in the PRICING dict
• Model-to-encoding mappings are in MODEL_ENCODING
• Reference: each provider's official pricing page
• Please include a test in tests/test_core.py verifying the new models appear in TokenManager.list_models()

I'll review PRs within 24h.