feat: add account-scoped filtering to /connz (Gap 10.2)

Add ConnzConnectionInfo, ConnzFilterResult, ConnzFilterOptions, and
ConnzFilter static class to Connz.cs, providing a pure unit-testable
layer for account-scoped filtering and pagination that mirrors the Go
server's /connz ?acc= query-parameter behaviour.  Ten new tests in
ConnzAccountFilterTests.cs cover FilterByAccount (match, no-match,
case-insensitive), ConnzFilterOptions.Parse (acc param, defaults,
offset/limit), and ApplyFilters (account filter, offset, limit,
no-filter pass-through).

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

@@ -0,0 +1,190 @@
+using NATS.Server.Monitoring;
+
+namespace NATS.Server.Tests.Monitoring;
+
+/// <summary>
+/// Unit tests for account-scoped /connz filtering (Gap 10.2).
+/// Exercises ConnzFilterOptions.Parse, ConnzFilter.FilterByAccount, and
+/// ConnzFilter.ApplyFilters in isolation — no running server required.
+/// Go reference: monitor_test.go — TestConnzFilterByAccount, TestConnzWithAccount.
+/// </summary>
+public class ConnzAccountFilterTests
+{
+    // -----------------------------------------------------------------------
+    // Helper — build a ConnzConnectionInfo with sensible defaults
+    // -----------------------------------------------------------------------
+
+    private static ConnzConnectionInfo MakeConn(
+        ulong clientId,
+        string? accountName,
+        string remoteAddress = "127.0.0.1:1234",
+        string? name = null,
+        long inMsgs = 0,
+        long outMsgs = 0,
+        long inBytes = 0,
+        long outBytes = 0) =>
+        new(clientId, remoteAddress, accountName, name, DateTime.UtcNow,
+            inMsgs, outMsgs, inBytes, outBytes);
+
+    // -----------------------------------------------------------------------
+    // FilterByAccount tests
+    // -----------------------------------------------------------------------
+
+    [Fact]
+    public void FilterByAccount_MatchingAccount_ReturnsFiltered()
+    {
+        // Go reference: monitor.go Connz() — "if opts.Account != "" { ... }" filter
+        var connections = new[]
+        {
+            MakeConn(1, "acctA"),
+            MakeConn(2, "acctB"),
+            MakeConn(3, "acctA"),
+        };
+
+        var result = ConnzFilter.FilterByAccount(connections, "acctA");
+
+        result.Count.ShouldBe(2);
+        result.ShouldAllBe(c => c.AccountName == "acctA");
+    }
+
+    [Fact]
+    public void FilterByAccount_NoMatch_ReturnsEmpty()
+    {
+        var connections = new[]
+        {
+            MakeConn(1, "acctA"),
+            MakeConn(2, "acctB"),
+        };
+
+        var result = ConnzFilter.FilterByAccount(connections, "acctC");
+
+        result.ShouldBeEmpty();
+    }
+
+    [Fact]
+    public void FilterByAccount_CaseInsensitive()
+    {
+        // Go reference: monitor.go — account name comparison is case-insensitive
+        var connections = new[]
+        {
+            MakeConn(1, "AcctA"),
+            MakeConn(2, "acctb"),
+        };
+
+        // Upper-case query against lower-case stored name
+        var lowerResult = ConnzFilter.FilterByAccount(connections, "accta");
+        lowerResult.Count.ShouldBe(1);
+        lowerResult[0].ClientId.ShouldBe(1UL);
+
+        // Lower-case query against mixed-case stored name
+        var upperResult = ConnzFilter.FilterByAccount(connections, "ACCTB");
+        upperResult.Count.ShouldBe(1);
+        upperResult[0].ClientId.ShouldBe(2UL);
+    }
+
+    // -----------------------------------------------------------------------
+    // ConnzFilterOptions.Parse tests
+    // -----------------------------------------------------------------------
+
+    [Fact]
+    public void Parse_WithAccParam_SetsAccountFilter()
+    {
+        var opts = ConnzFilterOptions.Parse("?acc=myAccount");
+
+        opts.AccountFilter.ShouldBe("myAccount");
+    }
+
+    [Fact]
+    public void Parse_WithoutParams_DefaultValues()
+    {
+        var opts = ConnzFilterOptions.Parse(null);
+
+        opts.AccountFilter.ShouldBeNull();
+        opts.StateFilter.ShouldBeNull();
+        opts.Offset.ShouldBe(0);
+        opts.Limit.ShouldBe(1024);
+    }
+
+    [Fact]
+    public void Parse_WithOffsetAndLimit_ParsesCorrectly()
+    {
+        var opts = ConnzFilterOptions.Parse("?acc=acctA&state=open&offset=10&limit=50");
+
+        opts.AccountFilter.ShouldBe("acctA");
+        opts.StateFilter.ShouldBe("open");
+        opts.Offset.ShouldBe(10);
+        opts.Limit.ShouldBe(50);
+    }
+
+    // -----------------------------------------------------------------------
+    // ApplyFilters tests
+    // -----------------------------------------------------------------------
+
+    [Fact]
+    public void ApplyFilters_WithAccountFilter_FiltersCorrectly()
+    {
+        var connections = new[]
+        {
+            MakeConn(1, "acctA"),
+            MakeConn(2, "acctB"),
+            MakeConn(3, "acctA"),
+        };
+
+        var opts = new ConnzFilterOptions { AccountFilter = "acctA" };
+        var result = ConnzFilter.ApplyFilters(connections, opts);
+
+        result.Total.ShouldBe(2);
+        result.Connections.Count.ShouldBe(2);
+        result.Connections.ShouldAllBe(c => c.AccountName == "acctA");
+    }
+
+    [Fact]
+    public void ApplyFilters_WithOffset_SkipsEntries()
+    {
+        var connections = Enumerable.Range(1, 5)
+            .Select(i => MakeConn((ulong)i, "acctA"))
+            .ToList();
+
+        var opts = new ConnzFilterOptions { Offset = 3, Limit = 10 };
+        var result = ConnzFilter.ApplyFilters(connections, opts);
+
+        result.Total.ShouldBe(5);
+        result.Connections.Count.ShouldBe(2);
+        result.Offset.ShouldBe(3);
+        // The paged items should be the last two (clientId 4 and 5)
+        result.Connections[0].ClientId.ShouldBe(4UL);
+        result.Connections[1].ClientId.ShouldBe(5UL);
+    }
+
+    [Fact]
+    public void ApplyFilters_WithLimit_CapsResults()
+    {
+        var connections = Enumerable.Range(1, 10)
+            .Select(i => MakeConn((ulong)i, "acctA"))
+            .ToList();
+
+        var opts = new ConnzFilterOptions { Limit = 3 };
+        var result = ConnzFilter.ApplyFilters(connections, opts);
+
+        result.Total.ShouldBe(10);
+        result.Connections.Count.ShouldBe(3);
+        result.Limit.ShouldBe(3);
+    }
+
+    [Fact]
+    public void ApplyFilters_NoFilters_ReturnsAll()
+    {
+        var connections = new[]
+        {
+            MakeConn(1, "acctA"),
+            MakeConn(2, "acctB"),
+            MakeConn(3, null),
+        };
+
+        var opts = new ConnzFilterOptions();
+        var result = ConnzFilter.ApplyFilters(connections, opts);
+
+        result.Total.ShouldBe(3);
+        result.Connections.Count.ShouldBe(3);
+    }
+}