[Bugfix][Attention] Keep model-dtype query for FlashInfer builders serving non-causal attention

[guan404ming]

Purpose

Fix serving an NVFP4 target with a DFlash drafter on SM100, which fails at startup with fp8 tensor core is not supported in fa2 backend because the ModelOpt checkpoint forces FP8 KV cache. Related: #48234, #46105.

• Keep queries in model dtype when a FlashInfer builder serves non-causal layers: trtllm-gen is causal-only, so those layers route to fa2, which cannot consume FP8 queries
• Causal FP8 paths (target model, MTP) unchanged; relax once FlashInfer's non-causal spec-decode kernels land ([Tracking][Spec Decode] DFlash Bring-Up Tracker #46105)

Test Plan

• Serve nvidia/Qwen3.6-27B-NVFP4 + z-lab/Qwen3.6-27B-DFlash on B200 with VLLM_USE_V2_MODEL_RUNNER=1, 8 concurrent chat streams for 5 minutes
• Serve the same target without spec decode as a causal regression check

Test Result

• Before: startup assert regardless of --kv-cache-dtype
• After: 116/116 requests OK, 0 errors, spec decoding active, ~382 tok/s
• Causal-only serving: unchanged

[claude]

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