feat: create NATS.Server.TestUtilities with shared helpers

Add shared test utility library with TestPortAllocator.GetFreePort() and
SocketTestHelper.ReadUntilAsync() to deduplicate helpers across test projects.
This is the foundation for splitting the monolithic test project into
feature-focused test projects.

tests/NATS.Server.TestUtilities/SocketTestHelper.cs

@@ -0,0 +1,21 @@
+using System.Net.Sockets;
+using System.Text;
+
+namespace NATS.Server.TestUtilities;
+
+public static class SocketTestHelper
+{
+    public static async Task<string> ReadUntilAsync(Socket sock, string expected, int timeoutMs = 5000)
+    {
+        using var cts = new CancellationTokenSource(timeoutMs);
+        var sb = new StringBuilder();
+        var buf = new byte[4096];
+        while (!sb.ToString().Contains(expected, StringComparison.Ordinal))
+        {
+            var n = await sock.ReceiveAsync(buf, SocketFlags.None, cts.Token);
+            if (n == 0) break;
+            sb.Append(Encoding.ASCII.GetString(buf, 0, n));
+        }
+        return sb.ToString();
+    }
+}