Remove unnecessary cast - GetSpecialType returns INamedTypeSymbol

Co-authored-by: YunchuWang <[email protected]>

Author: Copilot

src/Generators/AzureFunctions/DurableFunction.cs

@@ -146,7 +146,7 @@ public static bool TryParse(SemanticModel model, MethodDeclarationSyntax method,
 
             // For void returns, pass a dummy object type since the constructor needs an ITypeSymbol
             // but the ReturnsVoid flag will ensure it's not actually used for code generation
-            ITypeSymbol returnTypeForConstructor = returnSymbol ?? (ITypeSymbol)model.Compilation.GetSpecialType(SpecialType.System_Object);
+            ITypeSymbol returnTypeForConstructor = returnSymbol ?? model.Compilation.GetSpecialType(SpecialType.System_Object);
 
             function = new DurableFunction(fullTypeName!, name, kind, parameter, returnTypeForConstructor, returnsVoid, requiredNamespaces);
             return true;