### What this PR does / why we need it?
add ut for decorator.py/deepseek_mtp.py
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
CI passed with new tests
- vLLM version: v0.10.0
- vLLM main:
055bd3978e
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Signed-off-by: CaranLic <740821011@qq.com>