feat: port session 01 — Foundation Types (const, errors, proto, ring, rate_counter, sdm)

Ports server/const.go, errors.go, proto.go, ring.go, rate_counter.go, sdm.go.
- ServerConstants: all protocol constants and version info from const.go
- ServerErrors: ~60 sentinel exceptions plus errCtx/configErr/processConfigErr types
- ProtoWire: protobuf varint encode/decode helpers (proto.go)
- RateCounter: sliding-window rate limiter (rate_counter.go)
- ClosedRingBuffer: fixed-size ring buffer for /connz (ring.go)
- StreamDeletionMeta: SDM tracking for JetStream cluster consensus (sdm.go)
- 5 unit tests passing (errors, ring buffer, rate counter)
- errors_gen.go (code generator tool) and nkey.go Server methods marked n_a

dotnet/tests/ZB.MOM.NatsNet.Server.Tests/Internal/ClosedRingBufferTests.cs

@@ -0,0 +1,79 @@
+// Copyright 2018-2025 The NATS Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+using Shouldly;
+using ZB.MOM.NatsNet.Server.Internal;
+
+namespace ZB.MOM.NatsNet.Server.Tests.Internal;
+
+/// <summary>
+/// Tests for <see cref="ClosedRingBuffer"/>.
+/// Mirrors server/ring_test.go: TestRBAppendAndLenAndTotal (ID 2794)
+/// and TestRBclosedClients (ID 2795).
+/// </summary>
+public sealed class ClosedRingBufferTests
+{
+    [Fact]
+    public void AppendAndLenAndTotal_ShouldTrackCorrectly()
+    {
+        // Mirror: TestRBAppendAndLenAndTotal
+        var rb = new ClosedRingBuffer(10);
+
+        for (var i = 0; i < 5; i++)
+            rb.Append(new ClosedClient());
+
+        rb.Len().ShouldBe(5);
+        rb.TotalConns().ShouldBe(5UL);
+
+        for (var i = 0; i < 25; i++)
+            rb.Append(new ClosedClient());
+
+        rb.Len().ShouldBe(10);
+        rb.TotalConns().ShouldBe(30UL);
+    }
+
+    [Fact]
+    public void ClosedClients_ShouldReturnChronologicalOrder()
+    {
+        // Mirror: TestRBclosedClients
+        var rb = new ClosedRingBuffer(10);
+
+        // Build master list with identifiable user strings.
+        const int max = 100;
+        var master = new ClosedClient[max];
+        for (var i = 1; i <= max; i++)
+            master[i - 1] = new ClosedClient { User = i.ToString() };
+
+        var ui = 0;
+
+        void AddConn()
+        {
+            ui++;
+            rb.Append(new ClosedClient { User = ui.ToString() });
+        }
+
+        for (var i = 0; i < max; i++)
+        {
+            AddConn();
+
+            var ccs = rb.ClosedClients();
+            var start = (int)rb.TotalConns() - ccs.Length;
+            var ms = master[start..(start + ccs.Length)];
+
+            // Verify order matches master using User strings.
+            ccs.Length.ShouldBe(ms.Length);
+            for (var j = 0; j < ccs.Length; j++)
+                ccs[j]!.User.ShouldBe(ms[j].User, $"iteration {i}, slot {j}");
+        }
+    }
+}