docs: complete XML doc coverage (returns, summaries, inheritdoc)

Resolve all 622 issues flagged by the enhanced CommentChecker: add missing
<returns> tags (incl. the standard phrasing on non-generic Task methods),
add missing <summary> tags, and replace misused/redundant <inheritdoc/> on
members that override or implement nothing with real documentation.
Documentation-only — no behavior change; solution builds clean.

src/ZB.MOM.WW.ScadaBridge.NotificationService/SmtpErrorClassifier.cs

@@ -43,6 +43,7 @@ public static class SmtpErrorClassifier
     /// The token governing the send; a requested cancellation classifies as
     /// <see cref="SmtpErrorClass.Unknown"/> so the caller can re-throw it.
     /// </param>
+    /// <returns>The <see cref="SmtpErrorClass"/> describing whether the failure is transient, permanent, or unknown.</returns>
     public static SmtpErrorClass Classify(Exception ex, CancellationToken cancellationToken)
     {
         ArgumentNullException.ThrowIfNull(ex);
@@ -90,6 +91,7 @@ public static class SmtpErrorClassifier
     /// </summary>
     /// <param name="ex">The exception to classify.</param>
     /// <param name="cancellationToken">Cancellation token passed to <see cref="Classify"/>.</param>
+    /// <returns><c>true</c> when <see cref="Classify"/> returns <see cref="SmtpErrorClass.Transient"/>.</returns>
     public static bool IsTransient(Exception ex, CancellationToken cancellationToken)
         => Classify(ex, cancellationToken) == SmtpErrorClass.Transient;
 }