refactor: extract NATS.Server.Monitoring.Tests project

Move 39 monitoring, events, and system endpoint test files from
NATS.Server.Tests into a dedicated NATS.Server.Monitoring.Tests project.
Update namespaces, replace private GetFreePort/ReadUntilAsync with
TestUtilities shared helpers, add InternalsVisibleTo, and register
in the solution file. All 439 tests pass.

tests/NATS.Server.Monitoring.Tests/Monitoring/ClosedConnectionRingBufferTests.cs

@@ -0,0 +1,240 @@
+// Go reference: server/monitor.go — closedClients ring buffer, ClosedState tracking.
+// These tests verify the fixed-size ring buffer used to track recently closed connections
+// for the /connz?state=closed monitoring endpoint.
+
+using NATS.Server.Monitoring;
+
+namespace NATS.Server.Monitoring.Tests.Monitoring;
+
+public class ClosedConnectionRingBufferTests
+{
+    // -----------------------------------------------------------------------
+    // Helpers
+    // -----------------------------------------------------------------------
+
+    private static ClosedClient MakeEntry(ulong cid, string reason = "normal") => new()
+    {
+        Cid = cid,
+        Ip = "127.0.0.1",
+        Port = 4222,
+        Start = DateTime.UtcNow.AddSeconds(-10),
+        Stop = DateTime.UtcNow,
+        Reason = reason,
+    };
+
+    // -----------------------------------------------------------------------
+    // 1. Add_IncreasesCount
+    // -----------------------------------------------------------------------
+
+    /// <summary>
+    /// Adding entries should increase Count up to capacity.
+    /// </summary>
+    [Fact]
+    public void Add_IncreasesCount()
+    {
+        var buf = new ClosedConnectionRingBuffer(capacity: 10);
+
+        buf.Count.ShouldBe(0);
+
+        buf.Add(MakeEntry(1));
+        buf.Count.ShouldBe(1);
+
+        buf.Add(MakeEntry(2));
+        buf.Count.ShouldBe(2);
+
+        buf.Add(MakeEntry(3));
+        buf.Count.ShouldBe(3);
+    }
+
+    // -----------------------------------------------------------------------
+    // 2. Add_RingOverwrite_CapacityNotExceeded
+    // -----------------------------------------------------------------------
+
+    /// <summary>
+    /// When capacity is exceeded the count stays at capacity (oldest entry is overwritten).
+    /// </summary>
+    [Fact]
+    public void Add_RingOverwrite_CapacityNotExceeded()
+    {
+        const int capacity = 5;
+        var buf = new ClosedConnectionRingBuffer(capacity);
+
+        for (var i = 1; i <= capacity + 3; i++)
+            buf.Add(MakeEntry((ulong)i));
+
+        buf.Count.ShouldBe(capacity);
+    }
+
+    // -----------------------------------------------------------------------
+    // 3. GetAll_ReturnsNewestFirst
+    // -----------------------------------------------------------------------
+
+    /// <summary>
+    /// GetAll should return entries ordered newest-first.
+    /// </summary>
+    [Fact]
+    public void GetAll_ReturnsNewestFirst()
+    {
+        var buf = new ClosedConnectionRingBuffer(capacity: 10);
+
+        buf.Add(MakeEntry(1));
+        buf.Add(MakeEntry(2));
+        buf.Add(MakeEntry(3));
+
+        var all = buf.GetAll();
+
+        all.Count.ShouldBe(3);
+        all[0].Cid.ShouldBe(3UL); // newest
+        all[1].Cid.ShouldBe(2UL);
+        all[2].Cid.ShouldBe(1UL); // oldest
+    }
+
+    // -----------------------------------------------------------------------
+    // 4. GetAll_EmptyBuffer_ReturnsEmpty
+    // -----------------------------------------------------------------------
+
+    /// <summary>
+    /// GetAll on an empty buffer should return an empty list.
+    /// </summary>
+    [Fact]
+    public void GetAll_EmptyBuffer_ReturnsEmpty()
+    {
+        var buf = new ClosedConnectionRingBuffer(capacity: 10);
+
+        var all = buf.GetAll();
+
+        all.ShouldBeEmpty();
+    }
+
+    // -----------------------------------------------------------------------
+    // 5. GetRecent_ReturnsRequestedCount
+    // -----------------------------------------------------------------------
+
+    /// <summary>
+    /// GetRecent(n) where n &lt;= Count should return exactly n entries.
+    /// </summary>
+    [Fact]
+    public void GetRecent_ReturnsRequestedCount()
+    {
+        var buf = new ClosedConnectionRingBuffer(capacity: 10);
+        for (var i = 1; i <= 8; i++)
+            buf.Add(MakeEntry((ulong)i));
+
+        var recent = buf.GetRecent(3);
+
+        recent.Count.ShouldBe(3);
+        recent[0].Cid.ShouldBe(8UL); // newest
+        recent[1].Cid.ShouldBe(7UL);
+        recent[2].Cid.ShouldBe(6UL);
+    }
+
+    // -----------------------------------------------------------------------
+    // 6. GetRecent_LessThanAvailable_ReturnsAll
+    // -----------------------------------------------------------------------
+
+    /// <summary>
+    /// GetRecent(n) where n &gt; Count should return all available entries.
+    /// </summary>
+    [Fact]
+    public void GetRecent_LessThanAvailable_ReturnsAll()
+    {
+        var buf = new ClosedConnectionRingBuffer(capacity: 10);
+
+        buf.Add(MakeEntry(1));
+        buf.Add(MakeEntry(2));
+
+        var recent = buf.GetRecent(100);
+
+        recent.Count.ShouldBe(2);
+    }
+
+    // -----------------------------------------------------------------------
+    // 7. TotalClosed_TracksAllAdditions
+    // -----------------------------------------------------------------------
+
+    /// <summary>
+    /// TotalClosed should increment for every Add, even after the ring wraps around.
+    /// </summary>
+    [Fact]
+    public void TotalClosed_TracksAllAdditions()
+    {
+        const int capacity = 4;
+        var buf = new ClosedConnectionRingBuffer(capacity);
+
+        buf.TotalClosed.ShouldBe(0L);
+
+        for (var i = 1; i <= 10; i++)
+            buf.Add(MakeEntry((ulong)i));
+
+        buf.TotalClosed.ShouldBe(10L);
+        buf.Count.ShouldBe(capacity); // buffer is full but total reflects all 10
+    }
+
+    // -----------------------------------------------------------------------
+    // 8. Clear_ResetsCountAndBuffer
+    // -----------------------------------------------------------------------
+
+    /// <summary>
+    /// Clear should reset Count to zero and GetAll should return an empty list.
+    /// TotalClosed is intentionally not reset because it is a running lifetime counter.
+    /// </summary>
+    [Fact]
+    public void Clear_ResetsCountAndBuffer()
+    {
+        var buf = new ClosedConnectionRingBuffer(capacity: 10);
+
+        buf.Add(MakeEntry(1));
+        buf.Add(MakeEntry(2));
+        buf.Add(MakeEntry(3));
+        buf.TotalClosed.ShouldBe(3L);
+
+        buf.Clear();
+
+        buf.Count.ShouldBe(0);
+        buf.GetAll().ShouldBeEmpty();
+        // TotalClosed is a lifetime counter; it is not reset by Clear.
+        buf.TotalClosed.ShouldBe(3L);
+    }
+
+    // -----------------------------------------------------------------------
+    // 9. Capacity_ReturnsConfiguredSize
+    // -----------------------------------------------------------------------
+
+    /// <summary>
+    /// Capacity should reflect the value passed to the constructor.
+    /// </summary>
+    [Fact]
+    public void Capacity_ReturnsConfiguredSize()
+    {
+        var buf = new ClosedConnectionRingBuffer(capacity: 42);
+        buf.Capacity.ShouldBe(42);
+    }
+
+    // -----------------------------------------------------------------------
+    // 10. Add_WrapsCorrectly
+    // -----------------------------------------------------------------------
+
+    /// <summary>
+    /// After adding capacity+1 items the oldest entry (cid=1) should no longer be present,
+    /// and the buffer should contain the most recent 'capacity' items.
+    /// </summary>
+    [Fact]
+    public void Add_WrapsCorrectly()
+    {
+        const int capacity = 5;
+        var buf = new ClosedConnectionRingBuffer(capacity);
+
+        for (var i = 1; i <= capacity + 1; i++)
+            buf.Add(MakeEntry((ulong)i));
+
+        var all = buf.GetAll();
+
+        all.Count.ShouldBe(capacity);
+
+        // cid=1 (the oldest) should have been overwritten
+        all.Any(e => e.Cid == 1UL).ShouldBeFalse();
+
+        // The newest entry (cid=capacity+1) should be first
+        all[0].Cid.ShouldBe((ulong)(capacity + 1));
+    }
+}