fix: address second round of Copilot review feedback

- Replace null-forgiving operator on Marshal.PtrToStringUni with
  null-coalescing fallback (AddPveClusterMemberCmdlet.cs:64)
- GetNextId now throws InvalidOperationException instead of silently
  returning 0 when API response cannot be parsed

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

src/PSProxmoxVE.Core/Services/ClusterConfigService.cs

@@ -396,10 +396,11 @@ namespace PSProxmoxVE.Core.Services
             {
                 var response = client.GetAsync(resource).GetAwaiter().GetResult();
                 var data = JObject.Parse(response)["data"];
-                // The API returns the ID as a string, parse it to int
+                if (data == null)
-                if (data != null && int.TryParse(data.ToString(), out var id))
+                    throw new InvalidOperationException("API response for next VMID did not contain a 'data' field.");
+                if (int.TryParse(data.ToString(), out var id))
                     return id;
-                return 0;
+                throw new InvalidOperationException($"API returned unexpected next VMID value: '{data}'");
             }
             finally
             {

src/PSProxmoxVE/Cmdlets/Cluster/AddPveClusterMemberCmdlet.cs

@@ -61,7 +61,7 @@ namespace PSProxmoxVE.Cmdlets.Cluster
             try
             {
                 ptr = Marshal.SecureStringToGlobalAllocUnicode(Password);
-                var plainPassword = Marshal.PtrToStringUni(ptr)!;
+                var plainPassword = Marshal.PtrToStringUni(ptr) ?? string.Empty;
 
                 var linkDict = ParseLinks(Links);