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/ServiceCollectionExtensions.cs

@@ -17,6 +17,7 @@ public static class ServiceCollectionExtensions
     /// Registers the External System Gateway services, HTTP client factory, and options.
     /// </summary>
     /// <param name="services">The service collection to configure.</param>
+    /// <returns>The same <see cref="IServiceCollection"/> for chaining.</returns>
     public static IServiceCollection AddExternalSystemGateway(this IServiceCollection services)
     {
         services.AddOptions<ExternalSystemGatewayOptions>()
@@ -49,6 +50,7 @@ public static class ServiceCollectionExtensions
     /// Placeholder for External System Gateway Akka.NET actor registrations (handled in AkkaHostedService).
     /// </summary>
     /// <param name="services">The service collection to configure.</param>
+    /// <returns>The same <see cref="IServiceCollection"/> for chaining.</returns>
     public static IServiceCollection AddExternalSystemGatewayActors(this IServiceCollection services)
     {
         // WP-10: Actor registration happens in AkkaHostedService.