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/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);
 }