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.ExternalSystemGateway/ErrorClassifier.cs

@@ -17,6 +17,7 @@ public static class ErrorClassifier
     /// succeed and risks duplicate side effects.
     /// </summary>
     /// <param name="statusCode">The HTTP status code to classify.</param>
+    /// <returns><see langword="true"/> for 5xx, 408, or 429; <see langword="false"/> for all other status codes.</returns>
     public static bool IsTransient(HttpStatusCode statusCode)
     {
         var code = (int)statusCode;
@@ -27,6 +28,7 @@ public static class ErrorClassifier
     /// Determines whether an exception represents a transient failure.
     /// </summary>
     /// <param name="exception">The exception to classify.</param>
+    /// <returns><see langword="true"/> for connection/timeout/cancellation exceptions; <see langword="false"/> otherwise.</returns>
     public static bool IsTransient(Exception exception)
     {
         return exception is HttpRequestException
@@ -40,6 +42,7 @@ public static class ErrorClassifier
     /// </summary>
     /// <param name="message">Human-readable failure description.</param>
     /// <param name="inner">Optional inner exception that caused the transient failure.</param>
+    /// <returns>A new <see cref="TransientExternalSystemException"/> wrapping the given details.</returns>
     public static TransientExternalSystemException AsTransient(string message, Exception? inner = null)
     {
         return new TransientExternalSystemException(message, inner);