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.ClusterInfrastructure/ClusterOptionsValidator.cs

@@ -18,11 +18,7 @@ public sealed class ClusterOptionsValidator : OptionsValidatorBase<ClusterOption
         "keep-oldest"
     };
 
-    /// <summary>
-    /// Validates the cluster options, recording a failure if any critical settings are misconfigured.
-    /// </summary>
-    /// <param name="builder">The accumulator to record failures on.</param>
-    /// <param name="options">The cluster options to validate.</param>
+    /// <inheritdoc />
     protected override void Validate(ValidationBuilder builder, ClusterOptions options)
     {
         // CI-012: design doc states "both nodes are seed nodes — each node lists

src/ZB.MOM.WW.ScadaBridge.ClusterInfrastructure/ServiceCollectionExtensions.cs

@@ -23,6 +23,7 @@ public static class ServiceCollectionExtensions
     /// </para>
     /// </summary>
     /// <param name="services">The service collection to register into.</param>
+    /// <returns>The same <paramref name="services"/> instance, for call chaining.</returns>
     public static IServiceCollection AddClusterInfrastructure(this IServiceCollection services)
     {
         services.TryAddEnumerable(