### What this PR does / why we need it?
Replace text-match assertions with a two-tier logprob accuracy check:
- Prefill (token 0): assert token ID is identical between eager baseline
and compiled mode, then verify logprob matches within `atol`.
- Decode (tokens 1-2): if chosen tokens match, compare logprobs
directly; if they differ, cross-lookup the baseline token in the
compiled model's top-20 distribution and assert the assigned logprob is
within `decode_atol` (defaults to 2x atol). This tolerates minor argmax
drift caused by floating-point differences while still catching
distribution divergence.
### Does this PR introduce _any_ user-facing change?
### How was this patch tested?
- vLLM version: v0.17.0
- vLLM main:
8a680463fa
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Signed-off-by: wangli <wangli858794774@gmail.com>
75fae619d5