feat(vllm): macOS/Metal support via vllm-metal (MLX)

Add an additive Apple-Silicon path to the existing vllm Python backend so
vLLM runs on macOS via vllm-metal (github.com/vllm-project/vllm-metal).

Spike outcome (proven on a real M4 / macOS 26.5, Qwen3-0.6B):
- vllm-metal registers through vLLM's platform-plugin entry point
  (metal -> vllm_metal:register); MetalPlatform activates and runs on the
  GPU through MLX.
- LocalAI's backend.py is UNCHANGED: AsyncEngineArgs(...) ->
  AsyncLLMEngine.from_engine_args transparently resolves to vLLM 0.23's v1
  AsyncLLM MLX engine, and async generate produced correct output.
- backend.py is NOT touched: its only empty_cache() call is CUDA-only
  (guarded by torch.cuda.is_available()), so the benign shutdown-only
  "Allocator for mps is not a DeviceAllocator" noise comes from vLLM's
  internal EngineCore teardown, not from our code.

Changes (all gated behind a darwin condition; Linux/CUDA/ROCm/Intel paths
are byte-for-byte unchanged):
- install.sh: darwin branch forces PYTHON_VERSION=3.12 (vllm-metal
  requirement), creates/activates LocalAI's managed venv via ensureVenv,
  then reproduces vllm-metal's installer INTO that venv (build vLLM 0.23.0
  from the release source tarball against requirements/cpu.txt, then install
  the prebuilt vllm-metal wheel from its latest GitHub release), and runs
  runProtogen. installRequirements is skipped on darwin.
- backend-matrix.yml: add a vllm includeDarwin entry (mps, python).
- index.yaml: add metal capability + concrete metal-vllm /
  metal-vllm-development child entries mirroring the metal-kitten-tts
  template.

Version coupling: vllm-metal pins vLLM 0.23.0, equal to LocalAI's current
vllm pin. Bumping vllm must be coordinated with a supporting vllm-metal
release; documented in install.sh and requirements-cublas13-after.txt.

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Assisted-by: Claude:opus-4.8 [Claude Code]

.github/backend-matrix.yml

@@ -4974,6 +4974,13 @@ includeDarwin:
   - backend: "kitten-tts"
     tag-suffix: "-metal-darwin-arm64-kitten-tts"
     build-type: "mps"
+  # vLLM on Apple Silicon via vllm-metal (MLX). The install is custom
+  # (backend/python/vllm/install.sh has a darwin branch); lang stays python so
+  # backend_build_darwin.yml drives it through build-darwin-python-backend ->
+  # scripts/build/python-darwin.sh, which runs the backend's install.sh.
+  - backend: "vllm"
+    tag-suffix: "-metal-darwin-arm64-vllm"
+    build-type: "mps"
   - backend: "piper"
     tag-suffix: "-metal-darwin-arm64-piper"
     build-type: "metal"