fix: Add missing System.Linq using statement in PreferGenericFindAnalyzer

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

Author: Copilot

src/bunit.analyzers/Analyzers/PreferGenericFindAnalyzer.cs

@@ -1,4 +1,5 @@
 using System.Collections.Immutable;
+using System.Linq;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.CSharp.Syntax;