Route extension-block methods through DecompileExtension on click

When the C# 14 extension members tree feature is on, the marker
methods inside a static extension container surface as children of
the dedicated ExtensionTreeNode rather than as plain methods on the
outer class. Clicking one was running DecompileMethod, which renders
the lowered static method body, not the source-faithful
`extension(T) { ... }` block the user expects.

Match the WPF behaviour: when MethodTreeNode.Parent is an
ExtensionTreeNode and the active language is C#, dispatch to
CSharpLanguage.DecompileExtension(IMethod) so the whole declaration
emits as it was written. Other languages and non-extension methods
keep the existing DecompileMethod path.

Assisted-by: Claude:claude-opus-4-7:Claude Code

ILSpy/TreeNodes/MethodTreeNode.cs

@@ -79,7 +79,17 @@ namespace ILSpy.TreeNodes
 		public override bool ShowExpander => false;
 
 		public override void Decompile(Language language, ITextOutput output, DecompilationOptions options)
-			=> language.DecompileMethod(MethodDefinition, output, options);
+		{
+			// When the user clicks a method that lives under an ExtensionTreeNode (the C# 14
+			// extension-block container), emit the whole extension declaration via the
+			// dedicated DecompileExtension path so the output reads as source-faithful
+			// `extension(T) { ... }` syntax, not the lowered static method we'd get from
+			// DecompileMethod. Falls through to the regular method path for everything else.
+			if (Parent is ExtensionTreeNode && language is CSharpLanguage cs)
+				cs.DecompileExtension(MethodDefinition, output, options);
+			else
+				language.DecompileMethod(MethodDefinition, output, options);
+		}
 
 		public override bool IsPublicAPI => MethodDefinition.Accessibility switch {
 			Accessibility.Public or Accessibility.Protected or Accessibility.ProtectedOrInternal => true,