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/Client/PveHttpClient.cs

@@ -30,6 +30,10 @@ namespace PSProxmoxVE.Core.Client
         private readonly HttpClient _httpClient;
         private bool _disposed;
 
+        /// <summary>
+        /// Creates an HTTP client authenticated with the specified PVE session.
+        /// </summary>
+        /// <param name="session">The authenticated PVE session providing credentials and base URL.</param>
         public PveHttpClient(PveSession session)
         {
             _session = session ?? throw new ArgumentNullException(nameof(session));
@@ -394,6 +398,7 @@ namespace PSProxmoxVE.Core.Client
             return sb.ToString();
         }
 
+        /// <inheritdoc />
         public void Dispose()
         {
             if (!_disposed)