fix: resolve all C# compiler warnings (CS8604, CS8618, CS8603, CS1573, xUnit1012)

- CS8604: Use cmdlet's own Node property instead of nullable task.Node
  in WaitForTask calls across all VM cmdlets
- CS8618: Make PveSession.ServerVersion nullable (PveVersion?) since it
  is set post-construction by PveAuthenticator
- CS8603: Add null-forgiving operator where IsNullOrWhiteSpace guards
  guarantee non-null in PveHttpClient
- CS1573: Add XML param documentation to all public service methods
- xUnit1012: Make test parameters nullable (string?) for null test cases

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

src/PSProxmoxVE.Core/Client/PveHttpClient.cs

@@ -372,7 +372,7 @@ namespace PSProxmoxVE.Core.Client
                 // Some endpoints return a plain string in "message"
                 var message = json["message"]?.ToString();
                 if (!string.IsNullOrWhiteSpace(message))
-                    return message;
+                    return message!;
             }
             catch
             {