[atom-vllm] adapt DeepSeek V4 MTP for vLLM plugin

[whx-sjtu]

Summary

• Wire DeepSeek V4 MTP into the ATOM vLLM plugin, including draft model registration, target/draft sharing, V4 proxy cache handling, and MTP-specific metadata adaptation.
• Add a validation utility and focused tests for DeepSeek V4 MTP registration, hidden-state contracts, metrics parsing, and vLLM speculative decoding patches.

Test Result