Refactor exception handling and improve tests

- Updated `Slice` method in `ReadOnlyTensorSpan<T>` to use `ThrowHelper.ThrowIndexOutOfRangeException()` for consistent error handling.
- Removed unnecessary `using System.Reflection;` directive from `TensorSpanTests.cs`.
- Adjusted `TensorSpan_SliceTest` to call `TensorSpan_TestEnumerator(spanInt);` after defining `spanInt` and updated exception checks.
- Introduced a `value` variable in `TensorSpan_TestEnumerator<T>` to accurately restore the enumerator's current value for improved assertions.

src/libraries/System.Numerics.Tensors/src/System/Numerics/Tensors/netcore/ReadOnlyTensorSpan.cs

@@ -714,7 +714,7 @@ internal ReadOnlyTensorSpan<T> Slice(scoped ReadOnlySpan<nint> lengths)
         public ReadOnlyTensorSpan<T> Slice(params scoped ReadOnlySpan<NRange> ranges)
         {
             if (ranges.Length != Lengths.Length)
-                throw new ArgumentOutOfRangeException(nameof(ranges), "Number of dimensions to slice does not equal the number of dimensions in the span");
+                ThrowHelper.ThrowIndexOutOfRangeException();
 
             ReadOnlyTensorSpan<T> toReturn;
             scoped Span<nint> lengths;

src/libraries/System.Numerics.Tensors/tests/TensorSpanTests.cs

@@ -5,7 +5,6 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
-using System.Reflection;
 using System.Runtime.InteropServices;
 using Xunit;
 
@@ -1954,11 +1953,11 @@ public static void TensorSpan_SliceTest()
             int[] a = [1, 2, 3, 4, 5, 6, 7, 8, 9];
             int[] results = new int[9];
             TensorSpan<int> spanInt = a.AsTensorSpan(3, 3);
-            TensorSpan_TestEnumerator(spanInt);
 
             Assert.Throws<IndexOutOfRangeException>(() => a.AsTensorSpan(2, 3).Slice(0..1));
             Assert.Throws<IndexOutOfRangeException>(() => a.AsTensorSpan(2, 3).Slice(1..2));
             Assert.Throws<ArgumentOutOfRangeException>(() => a.AsTensorSpan(2, 3).Slice(0..1, 5..6));
+            TensorSpan_TestEnumerator(spanInt);
 
             var sp = spanInt.Slice(1..3, 1..3);
             Assert.Equal(5, sp[0, 0]);
@@ -2124,10 +2123,11 @@ private static void TensorSpan_TestEnumerator<T>(TensorSpan<T> span)
                 Assert.True(enumerator.MoveNext());
                 ref var current = ref enumerator.Current;
 
+                var value = current;
                 Assert.Equal(span[curIndexes], current);
                 current++;
                 Assert.Equal(span[curIndexes], current);
-                current--;
+                current = value;
                 Assert.Equal(span[curIndexes], current);
             }
             Assert.False(enumerator.MoveNext());