fix: bump flashinfer to 0.6.9 to match the V4-Flash MXFP4 minimum

[gvr13n]

Fresh installs cannot serve DeepSeek-V4-Flash MXFP4: mxfp4_deepseek.py requires flashinfer >= 0.6.9 (_MIN_FI_VERSION — mxfp8_quantize, trtllm_fp4_block_scale_routed_moe), but pyproject.toml pins flashinfer_python/flashinfer_cubin ==0.6.3.

The failure mode is nasty to diagnose: the quant plugin fails its import-time version check with only a warning, the MXFP4 MoE method never registers, and the server later dies with an unrelated-looking AssertionError: Hidden size mismatch in the generic triton fused_experts during cuda-graph capture. (Hit this twice on a working V4-Flash + kt-hybrid box after a routine reinstall silently downgraded flashinfer per the pin.)

This bumps the pins and the Dockerfile FLASHINFER_VERSION arg together, preserving the documented jit-cache alignment.

Possibly worth a follow-up: promote the import-time version warning to a hard error when a V4 MXFP4 checkpoint is actually being served, so the failure surfaces at the true cause.

[gemini-code-assist]

Code Review

This pull request updates the FLASHINFER_VERSION from 0.6.3 to 0.6.9 in the Dockerfile, and correspondingly bumps the flashinfer_python and flashinfer_cubin dependency versions to 0.6.9 in pyproject.toml. There are no review comments, and I have no feedback to provide.

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