fix: Fix compilation error in DecoratorGenerator - add missing namespace import (#94)

* Initial plan

* Fix CI build failure by adding missing using statement

Co-authored-by: JerrettDavis <2610199+JerrettDavis@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: JerrettDavis <2610199+JerrettDavis@users.noreply.github.com>

Author: Copilot

src/PatternKit.Generators/DecoratorGenerator.cs

@@ -1,4 +1,5 @@
 using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using PatternKit.Generators.Decorator;
 using System.Text;