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.HealthMonitoring/CentralHealthAggregator.cs

@@ -211,6 +211,7 @@ public class CentralHealthAggregator : BackgroundService, ICentralHealthAggregat
     /// full <c>OfflineTimeout / 2</c>.
     /// </summary>
     /// <param name="options">The health monitoring options to derive the interval from.</param>
+    /// <returns>Half the shorter of the two configured offline timeouts.</returns>
     internal static TimeSpan ComputeCheckInterval(HealthMonitoringOptions options)
     {
         var shorter = options.OfflineTimeout < options.CentralOfflineTimeout

src/ZB.MOM.WW.ScadaBridge.HealthMonitoring/HealthMonitoringOptionsValidator.cs

@@ -16,11 +16,7 @@ namespace ZB.MOM.WW.ScadaBridge.HealthMonitoring;
 /// </summary>
 public sealed class HealthMonitoringOptionsValidator : OptionsValidatorBase<HealthMonitoringOptions>
 {
-    /// <summary>
-    /// Validates the health monitoring options, recording a failure if any interval values are non-positive.
-    /// </summary>
-    /// <param name="builder">The accumulator to record failures on.</param>
-    /// <param name="options">The health monitoring options to validate.</param>
+    /// <inheritdoc />
     protected override void Validate(ValidationBuilder builder, HealthMonitoringOptions options)
     {
         builder.RequireThat(options.ReportInterval > TimeSpan.Zero,

src/ZB.MOM.WW.ScadaBridge.HealthMonitoring/ICentralHealthAggregator.cs

@@ -33,8 +33,10 @@ public interface ICentralHealthAggregator
     void MarkHeartbeat(string siteId, DateTimeOffset receivedAt);
 
     /// <summary>Returns a snapshot of all currently tracked site health states.</summary>
+    /// <returns>A dictionary mapping site id to its current <see cref="SiteHealthState"/>.</returns>
     IReadOnlyDictionary<string, SiteHealthState> GetAllSiteStates();
     /// <summary>Returns the current health state for the specified site, or null if not tracked.</summary>
     /// <param name="siteId">The string identifier of the site to look up.</param>
+    /// <returns>The <see cref="SiteHealthState"/> for the site, or null if not yet seen.</returns>
     SiteHealthState? GetSiteState(string siteId);
 }

src/ZB.MOM.WW.ScadaBridge.HealthMonitoring/IClusterNodeProvider.cs

@@ -9,6 +9,7 @@ namespace ZB.MOM.WW.ScadaBridge.HealthMonitoring;
 public interface IClusterNodeProvider
 {
     /// <summary>Returns the current status of all cluster nodes for the provider's role scope.</summary>
+    /// <returns>A read-only list of node status records for all known cluster nodes.</returns>
     IReadOnlyList<NodeStatus> GetClusterNodes();
 
     /// <summary>

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

@@ -11,6 +11,7 @@ public static class ServiceCollectionExtensions
     /// Call this on site nodes only. For central, call AddCentralHealthAggregation() instead.
     /// </summary>
     /// <param name="services">The DI service collection to register into.</param>
+    /// <returns>The same <paramref name="services"/> instance, for call chaining.</returns>
     public static IServiceCollection AddSiteHealthMonitoring(this IServiceCollection services)
     {
         AddOptionsValidation(services);
@@ -24,6 +25,7 @@ public static class ServiceCollectionExtensions
     /// Does not start the HealthReportSender — call AddSiteHealthMonitoring() on site nodes for that.
     /// </summary>
     /// <param name="services">The DI service collection to register into.</param>
+    /// <returns>The same <paramref name="services"/> instance, for call chaining.</returns>
     public static IServiceCollection AddHealthMonitoring(this IServiceCollection services)
     {
         AddOptionsValidation(services);
@@ -37,6 +39,7 @@ public static class ServiceCollectionExtensions
     /// for the central cluster so it appears on /monitoring/health.
     /// </summary>
     /// <param name="services">The DI service collection to register into.</param>
+    /// <returns>The same <paramref name="services"/> instance, for call chaining.</returns>
     public static IServiceCollection AddCentralHealthAggregation(this IServiceCollection services)
     {
         AddOptionsValidation(services);