Add IEnumerable tests for inputs

FFMpegCore.Test/ArgumentBuilderTest.cs

@@ -630,5 +630,68 @@ public void Builder_BuildString_MultiInput()
                 .Arguments;
             Assert.AreEqual($"-i \"1.mp3\" -i \"2.mp3\" -i \"3.mp3\" -i \"4.mp3\" -filter_complex \"[0:0][1:0][2:0][3:0]amix=inputs=4:duration=longest:dropout_transition=1:normalize=0[final]\" -map \"[final]\" -c:a libmp3lame -b:a 128k -ar 48000 -map_metadata -1 -ac 2 -write_xing 0 -id3v2_version 0 \"output.mp3\" -y", str);
         }
+        [TestMethod]
+        public void Pre_VerifyExists_AllFilesExist()
+        {
+            // Arrange
+            var filePaths = new List<string>
+            {
+                Path.GetTempFileName(),
+                Path.GetTempFileName(),
+                Path.GetTempFileName()
+            };
+            var argument = new MultiInputArgument(true, filePaths);
+            try
+            {
+                // Act & Assert
+                argument.Pre(); // No exception should be thrown
+            }
+            finally
+            {
+                // Cleanup
+                foreach (var filePath in filePaths)
+                {
+                    File.Delete(filePath);
+                }
+            }
+        }
+
+        [TestMethod]
+        public void Pre_VerifyExists_SomeFilesNotExist()
+        {
+            // Arrange
+            var filePaths = new List<string>
+            {
+                Path.GetTempFileName(),
+                "file2.mp4",
+                "file3.mp4"
+            };  
+            var argument = new MultiInputArgument(true, filePaths);
+            try
+            {
+                // Act & Assert
+                Assert.ThrowsException<FileNotFoundException>(() => argument.Pre());
+            }
+            finally
+            {
+                // Cleanup
+                File.Delete(filePaths[0]);
+            }
+        }
+
+        [TestMethod]
+        public void Pre_VerifyExists_NoFilesExist()
+        {
+            // Arrange
+            var filePaths = new List<string>
+            {
+                "file1.mp4",
+                "file2.mp4",
+                "file3.mp4"
+            };
+            var argument = new MultiInputArgument(true, filePaths);
+            // Act & Assert
+            Assert.ThrowsException<FileNotFoundException>(() => argument.Pre());
+        }
     }
 }