chore: ⬆️ Update mudler/vllm.cpp to e28ec46c6 (fix macOS -Werror build)

Bumps vllm.cpp to e28ec46c6 which fixes a -Wnull-conversion error in
qwen3_5.cpp:12483 that broke the macOS Metal CI build under -Werror.

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

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?=bc5dbc367489755749e5e92ea9e8d6445948a1ce
+VLLM_CPP_VERSION?=e28ec46c6b396df48f27f3a8a2e9bf3f30c5e1b8
 
 # 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