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/RateCounterTests.cs

@@ -0,0 +1,47 @@
+// Copyright 2021-2025 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="RateCounter"/>.
+/// Mirrors server/rate_counter_test.go: TestRateCounter (ID 2720).
+/// </summary>
+public sealed class RateCounterTests
+{
+    [Fact]
+    public async Task RateCounter_ShouldAllowUpToLimitThenBlockAndReset()
+    {
+        // Mirror: TestRateCounter
+        var counter = new RateCounter(10) { Interval = TimeSpan.FromMilliseconds(100) };
+
+        // First 10 calls should be allowed (counts 0–9 < limit 10).
+        for (var i = 0; i < 10; i++)
+            counter.Allow().ShouldBeTrue($"should allow on iteration {i}");
+
+        // Next 5 should be blocked.
+        for (var i = 0; i < 5; i++)
+            counter.Allow().ShouldBeFalse($"should not allow on iteration {i}");
+
+        // countBlocked returns and resets the blocked count.
+        counter.CountBlocked().ShouldBe(5UL);
+        counter.CountBlocked().ShouldBe(0UL);
+
+        // After the window expires, should allow again.
+        await Task.Delay(150);
+        counter.Allow().ShouldBeTrue("should allow after window expired");
+    }
+}