fix(vllm-cpp): pin MLX gate from mainline

The previous pin was a merge commit from the experimental C ABI v9 branch. Pin the same MLX prefill gate on upstream main so the backend build does not pull unrelated ABI v9 work into every platform variant.

Assisted-by: Codex:gpt-5 [systematic-debugging]

Author: localai-org-maint-bot

backend/go/vllm-cpp/Makefile

@@ -11,7 +11,7 @@ JOBS?=$(shell nproc --ignore=1 2>/dev/null || sysctl -n hw.ncpu 2>/dev/null || e
 
 # vllm.cpp version
 VLLM_CPP_REPO?=https://github.com/mudler/vllm.cpp
-VLLM_CPP_VERSION?=eec09bed5a03457837b499781c23d8e44f106813
+VLLM_CPP_VERSION?=59343930f37ba08abfccb8de544bc51fc3828b16
 
 # MLX GEMM provider (darwin/metal only; see the metal branch below for why).
 # Consumed as the prebuilt pip wheel: building MLX from source needs `xcrun