feat: port session 04 — Logging, Signals & Services

- NatsLogger.cs: INatsLogger interface (Noticef/Warnf/Fatalf/Errorf/Debugf/Tracef),
  ServerLogging state class with atomic debug/trace flags, rate-limited logging
  (RateLimitWarnf/RateLimitDebugf), error variants (Errors/Errorc/Errorsc),
  MicrosoftLoggerAdapter bridging to ILogger
- SignalHandler.cs: ProcessSignal (Unix kill via Process), CommandToUnixSignal mapping
  (Stop→SIGKILL, Quit→SIGINT, Reopen→SIGUSR1, Reload→SIGHUP), ResolvePids via pgrep,
  SetProcessName, Run/IsWindowsService stubs for non-Windows
- 11 tests (6 logger, 5 signal/service)
- WASM/Windows signal stubs already n/a
- All 141 tests pass (140 unit + 1 integration)
- DB: features 368/3673 complete, tests 155/3257 complete (9.6% overall)

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

@@ -0,0 +1,70 @@
+// Copyright 2012-2025 The NATS Authors
+// Licensed under the Apache License, Version 2.0
+
+using System.Runtime.InteropServices;
+using Shouldly;
+using ZB.MOM.NatsNet.Server.Internal;
+
+namespace ZB.MOM.NatsNet.Server.Tests.Internal;
+
+/// <summary>
+/// Tests for SignalHandler — mirrors tests from server/signal_test.go.
+/// </summary>
+public class SignalHandlerTests
+{
+    /// <summary>
+    /// Mirrors CommandToSignal mapping tests.
+    /// </summary>
+    [Fact] // T:3158
+    public void CommandToUnixSignal_ShouldMapCorrectly()
+    {
+        SignalHandler.CommandToUnixSignal(ServerCommand.Stop).ShouldBe(UnixSignal.SigKill);
+        SignalHandler.CommandToUnixSignal(ServerCommand.Quit).ShouldBe(UnixSignal.SigInt);
+        SignalHandler.CommandToUnixSignal(ServerCommand.Reopen).ShouldBe(UnixSignal.SigUsr1);
+        SignalHandler.CommandToUnixSignal(ServerCommand.Reload).ShouldBe(UnixSignal.SigHup);
+    }
+
+    /// <summary>
+    /// Mirrors SetProcessName test.
+    /// </summary>
+    [Fact] // T:3155
+    public void SetProcessName_ShouldNotThrow()
+    {
+        Should.NotThrow(() => SignalHandler.SetProcessName("test-server"));
+    }
+
+    /// <summary>
+    /// Verify IsWindowsService returns false on non-Windows.
+    /// </summary>
+    [Fact] // T:3149
+    public void IsWindowsService_ShouldReturnFalse()
+    {
+        if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
+            return; // Skip on Windows
+        SignalHandler.IsWindowsService().ShouldBeFalse();
+    }
+
+    /// <summary>
+    /// Mirrors Run — service.go Run() simply invokes the start function.
+    /// </summary>
+    [Fact] // T:3148
+    public void Run_ShouldInvokeStartAction()
+    {
+        var called = false;
+        SignalHandler.Run(() => called = true);
+        called.ShouldBeTrue();
+    }
+
+    /// <summary>
+    /// ProcessSignal with invalid PID expression should return error.
+    /// </summary>
+    [Fact] // T:3157
+    public void ProcessSignal_InvalidPid_ShouldReturnError()
+    {
+        if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
+            return; // Skip on Windows
+
+        var err = SignalHandler.ProcessSignal(ServerCommand.Stop, "not-a-pid");
+        err.ShouldNotBeNull();
+    }
+}