feat(worker): add LOCALAI_PREFETCH_MODELS for boot-time gallery prefetch (#10108)

In LocalAI distributed mode the master streams a model GGUF to a
worker on first inference. On bandwidth-constrained cluster networks
(libp2p circuit-v2 relays under NAT, double-NAT residential, slow
overlays) that transfer can be slow or unreliable — meanwhile each
worker's outbound internet is usually fine.

LOCALAI_PREFETCH_MODELS lets the operator name gallery model IDs to
download at worker boot, BEFORE the worker subscribes to backend.install
events. Reuses gallery.InstallModelFromGallery so the on-disk /models
layout matches what the master would have pushed, and the master can
still push files on demand if the gallery is unreachable at boot
(prefetch is non-fatal on every error path).

The installer is wrapped in a function-value indirection so tests can
swap a fake without touching the real gallery; production never
reassigns the binding.

Assisted-by: Claude:claude-opus-4-7

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