[Feat] DeepSeek support

[bboynton97]

Add deepseek/deepseek and groq/DeepSeek-R1-Distill-Llama-70b to the preferred models list

[srilaasya]

Deepseek AI sign up page is down. Tests for deepseek/ models pending.

[tcdent]

Openrouter hosts these, too. We don't currently handle the second slash in the name very well, so there's likely a bit more work to support it.

The string would look like: openrouter/deepseek/deepseek-r1

https://openrouter.ai/deepseek/deepseek-r1

[srilaasya]

Seems like a good alternative to deepseek as a provider. I'm going to hold off on this until we can support it. Meanwhile, I've updated the langgraph list of providers to add a deepseek endpoint.