fix(tests): replace flaky sleep with retry loop in SubscriptionIndexTests

Use a 2-second deadline retry loop instead of fixed Thread.Sleep(200)
for async cache sweep verification, mirroring the Go test pattern.

dotnet/tests/ZB.MOM.NatsNet.Server.Tests/Internal/DataStructures/SubscriptionIndexTests.cs

@@ -360,8 +360,11 @@ public class SubscriptionIndexTests
         for (int i = 0; i < 2 * SubscriptionIndex.SlCacheMax; i++)
             s.Match($"foo-{i}");
 
-        // Cache sweep runs async, wait briefly.
-        Thread.Sleep(200);
+        // Cache sweep runs async; mirror Go test's retry window.
+        var deadline = DateTime.UtcNow + TimeSpan.FromSeconds(2);
+        while (DateTime.UtcNow < deadline && s.CacheCount() > SubscriptionIndex.SlCacheMax)
+            Thread.Sleep(10);
+
         s.CacheCount().ShouldBeLessThanOrEqualTo(SubscriptionIndex.SlCacheMax);
 
         // Test wildcard cache update.