feat: add sort options to /connz (Gap 10.3)

Add ConnzSortOption enum (ConnectionId, Start, BytesTo, BytesFrom,
MsgsTo, MsgsFrom, Subscriptions, Pending, Uptime, Idle, LastActivity)
and ConnzSorter static class with Parse(string?) and Sort(IEnumerable<ConnInfo>,
ConnzSortOption, bool) methods.

Add SortBy and SortDescending properties to ConnzFilterOptions and wire
them through ConnzFilterOptions.Parse (?sort= and ?desc= query params).

Add 10 unit tests in ConnzSortTests covering Parse defaults, known values,
unknown string fallback, and sort ordering for key fields.

Go reference: server/monitor_sort_opts.go, server/monitor.go Connz().

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

@@ -0,0 +1,179 @@
+using NATS.Server.Monitoring;
+
+namespace NATS.Server.Tests.Monitoring;
+
+/// <summary>
+/// Unit tests for ConnzSortOption enum, ConnzSorter.Parse, and ConnzSorter.Sort.
+/// All tests exercise the types in isolation — no running server required.
+/// Go reference: monitor_test.go — TestConnzSortedByCid, TestConnzSortedByBytesTo,
+///               TestConnzSortedByMsgsFrom, TestConnzSortedByStart, monitor_sort_opts.go.
+/// </summary>
+public class ConnzSortTests
+{
+    // -----------------------------------------------------------------------
+    // Helper — build a ConnInfo with specific field values
+    // -----------------------------------------------------------------------
+
+    private static ConnInfo MakeConn(
+        ulong cid = 1,
+        long outBytes = 0,
+        long inBytes = 0,
+        long outMsgs = 0,
+        long inMsgs = 0,
+        uint numSubs = 0,
+        int pending = 0,
+        DateTime connectedAt = default,
+        DateTime lastActivity = default) =>
+        new()
+        {
+            Cid = cid,
+            OutBytes = outBytes,
+            InBytes = inBytes,
+            OutMsgs = outMsgs,
+            InMsgs = inMsgs,
+            NumSubs = numSubs,
+            Pending = pending,
+            Start = connectedAt == default ? DateTime.UtcNow : connectedAt,
+            LastActivity = lastActivity == default ? DateTime.UtcNow : lastActivity,
+        };
+
+    // -----------------------------------------------------------------------
+    // ConnzSorter.Parse tests
+    // -----------------------------------------------------------------------
+
+    [Fact]
+    public void Parse_ConnectionId_Default()
+    {
+        // Go reference: monitor_sort_opts.go — empty/unknown value defaults to CID sort
+        ConnzSorter.Parse(null).ShouldBe(ConnzSortOption.ConnectionId);
+        ConnzSorter.Parse("").ShouldBe(ConnzSortOption.ConnectionId);
+        ConnzSorter.Parse("   ").ShouldBe(ConnzSortOption.ConnectionId);
+        ConnzSorter.Parse("cid").ShouldBe(ConnzSortOption.ConnectionId);
+    }
+
+    [Fact]
+    public void Parse_BytesTo_Parsed()
+    {
+        // Go reference: monitor_sort_opts.go — "bytes_to" => ByBytesTo
+        ConnzSorter.Parse("bytes_to").ShouldBe(ConnzSortOption.BytesTo);
+        ConnzSorter.Parse("BYTES_TO").ShouldBe(ConnzSortOption.BytesTo);
+    }
+
+    [Fact]
+    public void Parse_MsgsFrom_Parsed()
+    {
+        // Go reference: monitor_sort_opts.go — "msgs_from" => ByMsgsFrom
+        ConnzSorter.Parse("msgs_from").ShouldBe(ConnzSortOption.MsgsFrom);
+        ConnzSorter.Parse("MSGS_FROM").ShouldBe(ConnzSortOption.MsgsFrom);
+    }
+
+    [Fact]
+    public void Parse_Start_Parsed()
+    {
+        // Go reference: monitor_sort_opts.go — "start" => ByStart
+        ConnzSorter.Parse("start").ShouldBe(ConnzSortOption.Start);
+        ConnzSorter.Parse("START").ShouldBe(ConnzSortOption.Start);
+    }
+
+    [Fact]
+    public void Parse_Unknown_ReturnsDefault()
+    {
+        // Go reference: monitor_sort_opts.go — unrecognised string falls back to CID
+        ConnzSorter.Parse("not_a_sort_key").ShouldBe(ConnzSortOption.ConnectionId);
+        ConnzSorter.Parse("xyz").ShouldBe(ConnzSortOption.ConnectionId);
+        ConnzSorter.Parse("RANDOM").ShouldBe(ConnzSortOption.ConnectionId);
+    }
+
+    // -----------------------------------------------------------------------
+    // ConnzSorter.Sort tests
+    // -----------------------------------------------------------------------
+
+    [Fact]
+    public void Sort_ByConnectionId_Ascending()
+    {
+        // Go reference: monitor_test.go TestConnzSortedByCid — CID ascending is default
+        var conns = new[]
+        {
+            MakeConn(cid: 3),
+            MakeConn(cid: 1),
+            MakeConn(cid: 2),
+        };
+
+        var result = ConnzSorter.Sort(conns, ConnzSortOption.ConnectionId);
+
+        result.Count.ShouldBe(3);
+        result[0].Cid.ShouldBe(1UL);
+        result[1].Cid.ShouldBe(2UL);
+        result[2].Cid.ShouldBe(3UL);
+    }
+
+    [Fact]
+    public void Sort_ByBytesTo_Descending()
+    {
+        // Go reference: monitor_test.go TestConnzSortedByBytesTo — OutBytes descending
+        var conns = new[]
+        {
+            MakeConn(cid: 1, outBytes: 100),
+            MakeConn(cid: 2, outBytes: 300),
+            MakeConn(cid: 3, outBytes: 200),
+        };
+
+        var result = ConnzSorter.Sort(conns, ConnzSortOption.BytesTo);
+
+        result.Count.ShouldBe(3);
+        result[0].OutBytes.ShouldBe(300);
+        result[1].OutBytes.ShouldBe(200);
+        result[2].OutBytes.ShouldBe(100);
+    }
+
+    [Fact]
+    public void Sort_ByStart_Ascending()
+    {
+        // Go reference: monitor_test.go TestConnzSortedByStart — ConnectedAt ascending
+        var t0 = new DateTime(2024, 1, 1, 0, 0, 0, DateTimeKind.Utc);
+        var t1 = t0.AddMinutes(1);
+        var t2 = t0.AddMinutes(2);
+
+        var conns = new[]
+        {
+            MakeConn(cid: 1, connectedAt: t2),
+            MakeConn(cid: 2, connectedAt: t0),
+            MakeConn(cid: 3, connectedAt: t1),
+        };
+
+        var result = ConnzSorter.Sort(conns, ConnzSortOption.Start);
+
+        result.Count.ShouldBe(3);
+        result[0].Start.ShouldBe(t0);
+        result[1].Start.ShouldBe(t1);
+        result[2].Start.ShouldBe(t2);
+    }
+
+    [Fact]
+    public void Sort_ByMsgsFrom_Descending()
+    {
+        // Go reference: monitor_test.go TestConnzSortedByMsgsFrom — InMsgs descending
+        var conns = new[]
+        {
+            MakeConn(cid: 1, inMsgs: 50),
+            MakeConn(cid: 2, inMsgs: 200),
+            MakeConn(cid: 3, inMsgs: 10),
+        };
+
+        var result = ConnzSorter.Sort(conns, ConnzSortOption.MsgsFrom);
+
+        result.Count.ShouldBe(3);
+        result[0].InMsgs.ShouldBe(200);
+        result[1].InMsgs.ShouldBe(50);
+        result[2].InMsgs.ShouldBe(10);
+    }
+
+    [Fact]
+    public void Sort_EmptyList_ReturnsEmpty()
+    {
+        // Sorting an empty input should not throw and should return an empty list.
+        var result = ConnzSorter.Sort([], ConnzSortOption.BytesTo);
+
+        result.ShouldBeEmpty();
+    }
+}