### What this PR does / why we need it?
[Nightly] Avoid max_model_len being smaller than the decoder prompt to
prevent single-node-accuray-tests from failing
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
- vLLM version: v0.12.0
- vLLM main:
ad32e3e19c
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Signed-off-by: ZT-AIA <1028681969@qq.com>
Signed-off-by: ZT-AIA <63220130+ZT-AIA@users.noreply.github.com>