Potential fix for pull request finding 'Missing Dispose call on local IDisposable'

Co-authored-by: Copilot Autofix powered by AI <223894421+github-code-quality[bot]@users.noreply.github.com>

test/WireMock.Net.Tests/Matchers/SystemTextJsonPartialMatcherTests.cs

@@ -58,9 +58,10 @@ public class SystemTextJsonPartialMatcherTests
     {
         // Assign
         var matcher = new SystemTextJsonPartialMatcher("{}");
+        using var stream = new MemoryStream();
 
         // Act
-        var result = matcher.IsMatch(new MemoryStream());
+        var result = matcher.IsMatch(stream);
 
         // Assert
         result.Score.Should().Be(MatchScores.Mismatch);