test: retry OPC UA host start on a port bind race (TOCTOU in AllocateFreePort)

The tests probe a free port (bind 0, release) and start the SDK server later; under a
highly parallel run another test can take the port in between and the SDK fails with
'Failed to establish tcp listener sockets for Ipv4 and IPv6.'. OpcUaHostStart.StartAsync
builds a fresh host + server on a fresh port per attempt (up to 5) when the failure is a
bind failure; all 45 start sites in OpcUaServer.Tests, OpcUaServer.IntegrationTests and
Runtime.Tests use it. Per-attempt GUID ApplicationUris are hoisted out of the host factory
because a changed URI against a reused PKI store fails with 'Cannot access private key'.

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe

tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/NodeManagerModelChangeOnAddTests.cs

@@ -108,31 +108,24 @@ public sealed class NodeManagerModelChangeOnAddTests : IDisposable
 
     private async Task<(OpcUaApplicationHost Host, OtOpcUaSdkServer Server)> BootAsync()
     {
-        var host = new OpcUaApplicationHost(
-            new OpcUaApplicationHostOptions
-            {
-                ApplicationName = "OtOpcUa.ModelChangeOnAddTest",
-                ApplicationUri = $"urn:OtOpcUa.ModelChangeOnAddTest:{Guid.NewGuid():N}",
-                OpcUaPort = AllocateFreePort(),
-                PublicHostname = "localhost",
-                PkiStoreRoot = _pkiRoot,
-            },
-            Microsoft.Extensions.Logging.Abstractions.NullLogger<OpcUaApplicationHost>.Instance);
-
-        var server = new OtOpcUaSdkServer();
-        await host.StartAsync(server, Ct);
+        var applicationUri = $"urn:OtOpcUa.ModelChangeOnAddTest:{Guid.NewGuid():N}";
+        var started = await OpcUaHostStart.StartAsync(
+            port => new OpcUaApplicationHost(
+                new OpcUaApplicationHostOptions
+                {
+                    ApplicationName = "OtOpcUa.ModelChangeOnAddTest",
+                    ApplicationUri = applicationUri,
+                    OpcUaPort = port,
+                    PublicHostname = "localhost",
+                    PkiStoreRoot = _pkiRoot,
+                },
+                Microsoft.Extensions.Logging.Abstractions.NullLogger<OpcUaApplicationHost>.Instance),
+            () => new OtOpcUaSdkServer(), Ct);
+        var host = started.Host;
+        var server = started.Server;
         return (host, server);
     }
 
-    private static int AllocateFreePort()
-    {
-        using var listener = new System.Net.Sockets.TcpListener(System.Net.IPAddress.Loopback, 0);
-        listener.Start();
-        var port = ((System.Net.IPEndPoint)listener.LocalEndpoint).Port;
-        listener.Stop();
-        return port;
-    }
-
     /// <summary>Cleans up the PKI root directory.</summary>
     public void Dispose()
     {