[Bugfix] Fix UT for the missing of log_stats in Engine

[gcanlin]

PLEASE FILL IN THE PR DESCRIPTION HERE ENSURING ALL CHECKLIST ITEMS (AT THE BOTTOM) HAVE BEEN CONSIDERED.

Purpose

Fix: https://buildkite.com/vllm/vllm-omni/builds/6416/steps/canvas

Because #2551 got merged without updating branch by the latest main, the #2519 introduced the new UT but #2551 didn't add the new field log_stats for the new UT.

Test Plan

Test Result

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