Pin disambiguators for constraints on sibling method type parameters

The chain cases in the fixture route T : TOuter through a class-level
type parameter; the variant where the dependency target is a sibling
method type parameter (M<T, U> with T : U) was uncovered. It pins the
same alignment from a different angle: csc rejects 'class' with CS8665
when U is merely class-constrained and requires 'default', but accepts
'class' when U carries a class-type constraint, matching what the
tri-state IsReferenceType derives. Both directions were verified
against csc before adding the expected output.

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

ICSharpCode.Decompiler.Tests/TestCases/Pretty/Issue3909.cs

@@ -207,6 +207,32 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			}
 		}
 
+		public abstract class MethodChainBase
+		{
+			public virtual T? DependentOnClassConstrained<T, U>(T? value, U other) where T : U where U : class
+			{
+				return value;
+			}
+
+			public virtual T? DependentOnClassType<T, U>(T? value, U other) where T : U where U : Node
+			{
+				return value;
+			}
+		}
+
+		public sealed class MethodChainDerived : MethodChainBase
+		{
+			public override T? DependentOnClassConstrained<T, U>(T? value, U other) where T : default
+			{
+				return value;
+			}
+
+			public override T? DependentOnClassType<T, U>(T? value, U other) where T : class
+			{
+				return value;
+			}
+		}
+
 		public class ContainerBase<TOuter> where TOuter : class
 		{
 			public virtual TOuter? Pick<TItem>(TItem item)