feat(boot): add server boot integration tests and fix ValueTask sync blocking

Fix Shutdown()/LameDuckMode() deadlock caused by calling .GetAwaiter().GetResult()
on pending ValueTask from Channel.ReadAsync(). ValueTask does not support synchronous
blocking — must convert via .AsTask() first. Add two integration tests validating the
full Start() → AcceptLoop → client connection → Shutdown lifecycle.

dotnet/src/ZB.MOM.NatsNet.Server/NatsServer.Lifecycle.cs

@@ -127,8 +127,9 @@ public sealed partial class NatsServer
         }
 
         // Wait for accept loops to exit.
+        // Must use .AsTask() because ValueTask cannot be synchronously blocked on.
         for (int i = 0; i < doneExpected; i++)
-            _done.Reader.ReadAsync().GetAwaiter().GetResult();
+            _done.Reader.ReadAsync().AsTask().GetAwaiter().GetResult();
 
         // Wait for all goroutines.
         _grWg.Wait();
@@ -655,8 +656,9 @@ public sealed partial class NatsServer
         ShutdownRaftNodes();
 
         // Wait for accept loops.
+        // Must use .AsTask() because ValueTask cannot be synchronously blocked on.
         for (int i = 0; i < expected; i++)
-            _ldmCh.Reader.ReadAsync().GetAwaiter().GetResult();
+            _ldmCh.Reader.ReadAsync().AsTask().GetAwaiter().GetResult();
 
         _mu.EnterWriteLock();
         var clients = new List<ClientConnection>(_clients.Values);