perf(library): avoid hash buffer copy

Author: Copilot

src/Microsoft.OpenApi/Models/OpenApiDocument.cs

@@ -577,7 +577,9 @@ public async Task<string> GetHashCodeAsync(CancellationToken cancellationToken =
             var hash = await SHA512.HashDataAsync(memoryStream, cancellationToken).ConfigureAwait(false);
 #else
             using HashAlgorithm sha = SHA512.Create();
-            var hash = sha.ComputeHash(memoryStream.ToArray());
+            var hash = memoryStream.TryGetBuffer(out ArraySegment<byte> buffer)
+                ? sha.ComputeHash(buffer.Array!, buffer.Offset, checked((int)memoryStream.Length))
+                : sha.ComputeHash(memoryStream.ToArray());
 #endif
 
             return ConvertByteArrayToString(hash ?? []);