test: add TLS rate limiter unit tests

tests/NATS.Server.Tests/TlsRateLimiterTests.cs

@@ -0,0 +1,49 @@
+using NATS.Server.Tls;
+
+namespace NATS.Server.Tests;
+
+public class TlsRateLimiterTests
+{
+    [Fact]
+    public async Task Rate_limiter_allows_configured_tokens_per_second()
+    {
+        using var limiter = new TlsRateLimiter(5);
+
+        // Should allow 5 tokens immediately
+        for (int i = 0; i < 5; i++)
+        {
+            using var cts = new CancellationTokenSource(100);
+            await limiter.WaitAsync(cts.Token); // Should not throw
+        }
+
+        // 6th token should block (no refill yet)
+        using var blockCts = new CancellationTokenSource(200);
+        var blocked = false;
+        try
+        {
+            await limiter.WaitAsync(blockCts.Token);
+        }
+        catch (OperationCanceledException)
+        {
+            blocked = true;
+        }
+        blocked.ShouldBeTrue("6th token should be blocked before refill");
+    }
+
+    [Fact]
+    public async Task Rate_limiter_refills_after_one_second()
+    {
+        using var limiter = new TlsRateLimiter(2);
+
+        // Consume all tokens
+        await limiter.WaitAsync(CancellationToken.None);
+        await limiter.WaitAsync(CancellationToken.None);
+
+        // Wait for refill
+        await Task.Delay(1200);
+
+        // Should have tokens again
+        using var cts = new CancellationTokenSource(200);
+        await limiter.WaitAsync(cts.Token); // Should not throw
+    }
+}