Refactor SurfaceCacheValidator to simplify oldest entry calculation

Add global using for Xunit in test project

Enhance ImportValidatorTests with async validation and quarantine checks

Implement FileSystemQuarantineServiceTests for quarantine functionality

Add integration tests for ImportValidator to check monotonicity

Create BundleVersionTests to validate version parsing and comparison logic

Implement VersionMonotonicityCheckerTests for monotonicity checks and activation logic

src/Signals/StellaOps.Signals/Services/CallgraphIngestionService.cs

@@ -330,20 +330,6 @@ internal sealed class CallgraphIngestionService : ICallgraphIngestionService
         return ordered.ToString();
     }
 
-    private static string JoinDict(IReadOnlyDictionary<string, string>? values)
-    {
-        if (values is null)
-        {
-            return string.Empty;
-        }
-
-        var ordered = new StringBuilder();
-        foreach (var kv in values.OrderBy(k => k.Key, StringComparer.Ordinal))
-        {
-            ordered.Append(kv.Key).Append('=').Append(kv.Value).Append(';');
-        }
-        return ordered.ToString();
-    }
 }
 
 /// <summary>