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/Foundation/ServerErrorsTests.cs

@@ -0,0 +1,87 @@
+// Copyright 2020-2025 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;
+
+namespace ZB.MOM.NatsNet.Server.Tests.Foundation;
+
+/// <summary>
+/// Tests for <see cref="ErrorCtx"/> and <see cref="ErrorContextHelper"/>.
+/// Mirrors server/errors_test.go: TestErrCtx (ID 297) and TestErrCtxWrapped (ID 298).
+/// </summary>
+public sealed class ServerErrorsTests
+{
+    [Fact]
+    public void ErrCtx_ShouldPreserveOriginalMessageAndAddContext()
+    {
+        // Mirror: TestErrCtx
+        var ctx = "Extra context information";
+        var e = ErrorContextHelper.NewErrorCtx(ServerErrors.ErrWrongGateway, "{0}", ctx);
+
+        // Message should match the underlying error.
+        e.Message.ShouldBe(ServerErrors.ErrWrongGateway.Message);
+
+        // Must not be reference-equal to the sentinel.
+        e.ShouldNotBeSameAs(ServerErrors.ErrWrongGateway);
+
+        // ErrorIs should find the sentinel in the chain.
+        ErrorContextHelper.ErrorIs(e, ServerErrors.ErrWrongGateway).ShouldBeTrue();
+
+        // UnpackIfErrorCtx on a non-ctx error returns Message unchanged.
+        ErrorContextHelper.UnpackIfErrorCtx(ServerErrors.ErrWrongGateway)
+            .ShouldBe(ServerErrors.ErrWrongGateway.Message);
+
+        // UnpackIfErrorCtx should start with the original error message.
+        var trace = ErrorContextHelper.UnpackIfErrorCtx(e);
+        trace.ShouldStartWith(ServerErrors.ErrWrongGateway.Message);
+
+        // And end with the context string.
+        trace.ShouldEndWith(ctx);
+    }
+
+    [Fact]
+    public void ErrCtxWrapped_ShouldContainAllContextLayers()
+    {
+        // Mirror: TestErrCtxWrapped
+        var ctxO = "Original Ctx";
+        var eO = ErrorContextHelper.NewErrorCtx(ServerErrors.ErrWrongGateway, "{0}", ctxO);
+
+        var ctx = "Extra context information";
+        var e = ErrorContextHelper.NewErrorCtx(eO, "{0}", ctx);
+
+        // Message should still match the underlying error.
+        e.Message.ShouldBe(ServerErrors.ErrWrongGateway.Message);
+
+        // Must not be reference-equal to the sentinel.
+        e.ShouldNotBeSameAs(ServerErrors.ErrWrongGateway);
+
+        // ErrorIs should walk the chain.
+        ErrorContextHelper.ErrorIs(e, ServerErrors.ErrWrongGateway).ShouldBeTrue();
+
+        // UnpackIfErrorCtx on a non-ctx error returns Message unchanged.
+        ErrorContextHelper.UnpackIfErrorCtx(ServerErrors.ErrWrongGateway)
+            .ShouldBe(ServerErrors.ErrWrongGateway.Message);
+
+        var trace = ErrorContextHelper.UnpackIfErrorCtx(e);
+
+        // Must start with the original error.
+        trace.ShouldStartWith(ServerErrors.ErrWrongGateway.Message);
+
+        // Must end with the outermost context.
+        trace.ShouldEndWith(ctx);
+
+        // Must also contain the inner context.
+        trace.ShouldContain(ctxO);
+    }
+}