feat: migration to mlx-swift-lm v3

MLXServerTests/Server/TokenPrefixCacheTests.swift

@@ -388,4 +388,10 @@ private final class TestTrimRecordingCache: KVCache {
     ) -> MLXFast.ScaledDotProductAttentionMaskMode {
         .none
     }
+
+    func copy() -> any KVCache {
+        let c = TestTrimRecordingCache(offset: offset, trimmable: trimmable)
+        c.state = state
+        return c
+    }
 }