docs: add XML doc comments to Core public API and remove CS1591 suppression

- Add missing XML doc comments to all public types and members in
  PSProxmoxVE.Core (Authentication, Exceptions, Models, Client)
- Remove CS1591 suppression from PSProxmoxVE.Core.csproj
- Build succeeds with 0 warnings

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

src/PSProxmoxVE.Core/Models/Vms/PveTaskLog.cs

@@ -16,6 +16,7 @@ namespace PSProxmoxVE.Core.Models.Vms
         [JsonProperty("t")]
         public string Text { get; set; } = string.Empty;
 
+        /// <inheritdoc />
         public override string ToString() => $"{LineNumber,4}: {Text}";
     }
 }