Fix StringWriter disposal in TraitMapperTests

Co-authored-by: IhateTrains <29546927+IhateTrains@users.noreply.github.com>

Author: Copilot

ImperatorToCK3.UnitTests/Mappers/Trait/TraitMapperTests.cs

@@ -134,7 +134,7 @@ public void UnmappedImperatorTraitsAreLoggedUnlessExplicitlyDropped() {
 		var mapper = new TraitMapper(tempTestFile, ck3ModFS);
 
 		var irModFS = new ModFilesystem("TestFiles/Imperator/game", new List<Mod>());
-		var output = new StringWriter();
+		using var output = new StringWriter();
 		Console.SetOut(output);
 
 		mapper.LogUnmappedImperatorTraits(irModFS);