Add client-side failover to CLI tool for redundancy testing

All commands gain --failover-urls (-F) to specify alternate endpoints.
Short-lived commands try each URL in order on initial connect. The
subscribe command monitors KeepAlive and automatically reconnects to
the next available server, re-creating the subscription on failover.
Verified with live service start/stop: primary down triggers failover
to secondary, primary restart allows failback.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tools/opcuacli-dotnet/Commands/ConnectCommand.cs

@@ -22,13 +22,18 @@ public class ConnectCommand : ICommand
     [CommandOption("security", 'S', Description = "Transport security: none, sign, encrypt (default: none)")]
     public string Security { get; init; } = "none";
 
+    [CommandOption("failover-urls", 'F', Description = "Comma-separated failover endpoint URLs for redundancy")]
+    public string? FailoverUrls { get; init; }
+
     /// <summary>
     /// Connects to the OPC UA endpoint and prints the resolved server metadata.
     /// </summary>
     /// <param name="console">The console used to report connection results.</param>
     public async ValueTask ExecuteAsync(IConsole console)
     {
-        using var session = await OpcUaHelper.ConnectAsync(Url, Username, Password, Security);
+        var urls = FailoverUrlParser.Parse(Url, FailoverUrls);
+        using var failover = new OpcUaFailoverHelper(urls, Username, Password, Security);
+        using var session = await failover.ConnectAsync();
         await console.Output.WriteLineAsync($"Connected to: {session.Endpoint.EndpointUrl}");
         await console.Output.WriteLineAsync($"Server: {session.Endpoint.Server!.ApplicationName}");
         await console.Output.WriteLineAsync($"Security Mode: {session.Endpoint.SecurityMode}");