flashinfer JIT-compiles kernels with ninja at startup, which fails on hosts without a
build toolchain wired (exit 127). Default it off so vLLM uses FLASH_ATTN + native sampler
out of the box; overridable via vllm.extra_env. Verified end-to-end: VllmBackend served
Qwen2.5-0.5B and answered a chat with token usage.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Mw2KQiswmD69T45fTfjKwW