chore: add SlopwatchSuppressAttribute for cluster test suppressions

tests/NATS.E2E.Cluster.Tests/SlopwatchSuppressAttribute.cs

@@ -0,0 +1,12 @@
+// Marker attribute recognised by the slopwatch static-analysis tool.
+// Apply to a test method to suppress a specific slopwatch rule violation.
+// The justification must be 20+ characters explaining why the suppression is intentional.
+
+namespace NATS.E2E.Cluster.Tests;
+
+[AttributeUsage(AttributeTargets.Method, AllowMultiple = true)]
+public sealed class SlopwatchSuppressAttribute(string ruleId, string justification) : Attribute
+{
+    public string RuleId { get; } = ruleId;
+    public string Justification { get; } = justification;
+}