Fix StringWriter disposal in TraitMapperTests (#2907)

The `StringWriter` in
`UnmappedImperatorTraitsAreLoggedUnlessExplicitlyDropped` test was not
being disposed, violating IDisposable patterns.

**Changes:**
- Added `using` declaration to ensure `StringWriter` is properly
disposed after test execution

```csharp
// Before
var output = new StringWriter();
Console.SetOut(output);

// After
using var output = new StringWriter();
Console.SetOut(output);
```

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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);