Add test for out-var reference nested within another argument

CS8196 also fires when the second reference sits inside a nested call
in another argument of the declaring call, e.g.
OutAndValue(out var a, UseAndReturn(out a)). The existing check already
covers this because it walks all descendants of the sibling arguments;
this fixture pins that behavior.

Assisted-by: Claude:claude-fable-5:Claude Code

ICSharpCode.Decompiler.Tests/TestCases/Pretty/OutVariables.cs

@@ -111,5 +111,24 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			GetTwo(out int a, out a);
 			return a;
 		}
+
+		private static void OutAndValue(out int a, int b)
+		{
+			a = b;
+		}
+
+		private static int UseAndReturn(out int a)
+		{
+			a = 1;
+			return a;
+		}
+
+		public static void SameVariableUsedInNestedCallArgument()
+		{
+			// CS8196 also applies when the second reference is nested within
+			// another argument of the declaring call.
+			OutAndValue(out int a, UseAndReturn(out a));
+			Console.WriteLine(a);
+		}
 	}
 }