feat(models): add deepseek model provider (#2307)

This feature #2211 makes DeepSeek a first-class OpenAI-compatible model
provider in AgentScope Java.
Users can resolve DeepSeek models with `deepseek:<model>` through
`ModelRegistry`, use
`DEEPSEEK_API_KEY` as the standard credential fallback, and get
DeepSeek-specific request
formatting by default without manually wiring `OpenAIChatModel`, base
URL, or formatter settings.

The implementation keeps DeepSeek on the existing OpenAI-compatible
transport stack while adding a
dedicated provider, compatibility-package formatters, focused test
coverage, and user-facing docs.