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/ServiceCollectionExtensions.cs

@@ -14,6 +14,7 @@ public static class ServiceCollectionExtensions
     /// was removed (NS-019). Notification dispatch lives in <c>ZB.MOM.WW.ScadaBridge.NotificationOutbox</c>.
     /// </summary>
     /// <param name="services">The service collection to register into.</param>
+    /// <returns>The same <paramref name="services"/> collection, for chaining.</returns>
     public static IServiceCollection AddNotificationService(this IServiceCollection services)
     {
         services.AddOptions<NotificationOptions>()
@@ -30,6 +31,7 @@ public static class ServiceCollectionExtensions
     /// Registers Akka.NET actors for the notification service (placeholder for actor registration).
     /// </summary>
     /// <param name="services">The service collection to register into.</param>
+    /// <returns>The same <paramref name="services"/> collection, for chaining.</returns>
     public static IServiceCollection AddNotificationServiceActors(this IServiceCollection services)
     {
         // Actor registration happens in AkkaHostedService.