fix(library): hash only written buffer bytes

Author: Copilot

src/Microsoft.OpenApi/Models/OpenApiDocument.cs

@@ -577,8 +577,8 @@ public async Task<string> GetHashCodeAsync(CancellationToken cancellationToken =
             var hash = await SHA512.HashDataAsync(memoryStream, cancellationToken).ConfigureAwait(false);
 #else
             using HashAlgorithm sha = SHA512.Create();
-            var hash = memoryStream.TryGetBuffer(out ArraySegment<byte> buffer)
-                ? sha.ComputeHash(buffer.Array!, buffer.Offset, checked((int)memoryStream.Length))
+            var hash = memoryStream.TryGetBuffer(out ArraySegment<byte> buffer) && buffer.Array is byte[] array
+                ? sha.ComputeHash(array, buffer.Offset, buffer.Count)
                 : sha.ComputeHash(memoryStream.ToArray());
 #endif