Made File.Handle a non-copyable struct in the library to adopt Swift 6.4.

Tests/PlaydateKit/MockPlaydate.swift

@@ -422,6 +422,19 @@ enum Mock {
     private static func installJSON() {
         jsonAPI.initialize(to: playdate_json())
 
+        // Pulls one chunk through the reader, as the OS would, and decodes
+        // it as `null`.
+        jsonAPI.pointee.decode = { _, reader, outval in
+            var buffer = [UInt8](repeating: 0, count: 16)
+            let count = buffer.withUnsafeMutableBufferPointer { buffer in
+                reader.read?(reader.userdata, buffer.baseAddress, Int32(buffer.count)) ?? -1
+            }
+            Mock.record("decode(\(count))")
+            outval?.pointee = json_value()
+            outval?.pointee.type = CChar(kJSONNull.rawValue)
+            return 1
+        }
+
         jsonAPI.pointee.decodeString = { decoder, _, outval in
             guard let decoder else { return 0 }