fix(ci): bump mlx-rs git rev to f8cfdd88 (CMake shallow-clone fix)

mlx-rs HEAD now carries the mlx-c CMakeLists.txt update that switches
the MLX C++ FetchContent_Declare GIT_TAG from a commit hash to the
"lumen-rs-patches" branch name. Required for CI builds since CMake's
default shallow git clone cannot resolve a bare commit hash via
GitHub's smart-http transport.

Author: rabbitson87

crates/lumen-mlx/Cargo.toml

@@ -31,10 +31,10 @@ lumen-core = { path = "../lumen-core" }
 # squashed commit on top of upstream's mlx-rs main). Local devs can
 # override to a sibling `../../../mlx-rs` checkout via the workspace
 # `[patch]` section in the top-level Cargo.toml.
-mlx-rs = { git = "https://github.com/rabbitson87/mlx-rs", rev = "382a5f33d223003c06e79bb0a8b5d27f7e0ff8bc", optional = true, default-features = false, features = [
+mlx-rs = { git = "https://github.com/rabbitson87/mlx-rs", rev = "f8cfdd8830d675ae7bbe5fbfb78ea6ea4478278c", optional = true, default-features = false, features = [
     "accelerate",
 ] }
-mlx-sys = { git = "https://github.com/rabbitson87/mlx-rs", rev = "382a5f33d223003c06e79bb0a8b5d27f7e0ff8bc", optional = true }
+mlx-sys = { git = "https://github.com/rabbitson87/mlx-rs", rev = "f8cfdd8830d675ae7bbe5fbfb78ea6ea4478278c", optional = true }
 
 # Phase 1.8 M1.5 — direct Metal kernel dispatch against mlx-allocated buffers.
 # Uses objc2 + objc2-metal for raw MTLDevice / MTLCommandQueue / pipeline
@@ -65,3 +65,4 @@ mlx-native-metal = ["mlx-native", "mlx-rs/metal"]
 
 [dev-dependencies]
 tokio = { version = "1.52.3", features = ["rt-multi-thread", "macros"] }
+tempfile = "3.10"