feat(cute_dsl/moe): re-enable use_cold_l2_cache in CuteDslMoEWrapper TuningConfig (#3384)

## 📌 Description

Sets `use_cold_l2_cache=True` on the autotuner `TuningConfig` in
`flashinfer/fused_moe/cute_dsl/tuner.py`, matching TRT-LLM's
`CuteDslFusedMoENvfp4Runner.tuning_config`. With cold-L2 ON, the
autotuner flushes L2 between profile iterations and measures
conservative timings, so the picked tactic is robustly fast under
cold-cache conditions; without it, back-to-back iterations of the same
tactic benefit from L2-hit reuse and bias the pick toward tactics that
look fast during profiling but aren't faster in production.

## 🔍 Related Issues

https://github.com/flashinfer-ai/flashinfer/pull/3286
https://github.com/flashinfer-ai/flashinfer/pull/3340

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## 🧪 Tests

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## Summary by CodeRabbit

* **Bug Fixes**
* Improved tuner's cold L2 cache measurement behavior for more accurate
performance profiling.

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>