Fixed MemoryStream seek to end validation (#3543)

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## Fixes #3536
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## PR Type
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- Bugfix
<!-- - Feature -->
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## What is the current behavior?
<!-- Please describe the current behavior that you are modifying, or link to a relevant issue. -->
Seeking to the end of a `Stream` from a `Memory<T>` or other types in the `HighPerformance` package throws an exception.

## What is the new behavior?
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Seeking to the end of the stream is now allowed. This is consistent to other `Stream` types in the BCL.

## PR Checklist

Please check if your PR fulfills the following requirements:

- [X] Tested code with current [supported SDKs](../readme.md#supported)
- [ ] ~~Pull Request has been submitted to the documentation repository [instructions](..\contributing.md#docs). Link: <!-- docs PR link -->~~
- [ ] ~~Sample in sample app has been added / updated (for bug fixes / features)~~
    - [ ] ~~Icon has been created (if new sample) following the [Thumbnail Style Guide and templates](https://github.com/windows-toolkit/WindowsCommunityToolkit-design-assets)~~
- [X] Tests for the changes have been added (for bug fixes / features) (if applicable)
- [X] Header has been added to all new source files (run *build/UpdateHeaders.bat*)
- [X] Contains **NO** breaking changes

Author: msftbot[bot]

Microsoft.Toolkit.HighPerformance/Streams/MemoryStream.Validate.cs

@@ -13,14 +13,14 @@ namespace Microsoft.Toolkit.HighPerformance.Streams
     internal static partial class MemoryStream
     {
         /// <summary>
-        /// Validates the <see cref="Stream.Position"/> argument.
+        /// Validates the <see cref="Stream.Position"/> argument (it needs to be in the [0, length]) range.
         /// </summary>
         /// <param name="position">The new <see cref="Stream.Position"/> value being set.</param>
         /// <param name="length">The maximum length of the target <see cref="Stream"/>.</param>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public static void ValidatePosition(long position, int length)
         {
-            if ((ulong)position >= (ulong)length)
+            if ((ulong)position > (ulong)length)
             {
                 ThrowArgumentOutOfRangeExceptionForPosition();
             }

UnitTests/UnitTests.HighPerformance.Shared/Streams/Test_MemoryStream.cs

@@ -78,6 +78,42 @@ public void Test_MemoryStream_Seek()
             Assert.AreEqual(stream.Position, 32);
         }
 
+        // See https://github.com/windows-toolkit/WindowsCommunityToolkit/issues/3536
+        [TestCategory("MemoryStream")]
+        [TestMethod]
+        public void Test_MemoryStream_WriteToEndAndRefreshPosition()
+        {
+            byte[]
+                array = new byte[10],
+                temp = new byte[1];
+            ReadOnlyMemory<byte> memory = array;
+
+            using var stream = memory.AsStream();
+
+            for (int i = 0; i < array.Length; i++)
+            {
+                int read = stream.Read(temp, 0, 1);
+
+                Assert.AreEqual(read, 1);
+                Assert.AreEqual(stream.Position, i + 1);
+            }
+
+            Assert.AreEqual(stream.Position, array.Length);
+
+            // These should not throw, seeking to the end is valid
+            stream.Position = stream.Position;
+            Assert.AreEqual(stream.Position, array.Length);
+
+            stream.Seek(array.Length, SeekOrigin.Begin);
+            Assert.AreEqual(stream.Position, array.Length);
+
+            stream.Seek(0, SeekOrigin.Current);
+            Assert.AreEqual(stream.Position, array.Length);
+
+            stream.Seek(0, SeekOrigin.End);
+            Assert.AreEqual(stream.Position, array.Length);
+        }
+
         [TestCategory("MemoryStream")]
         [TestMethod]
         public void Test_MemoryStream_ReadWrite_Array()