fix(health-monitoring): resolve HealthMonitoring-004,006,010,011,012 — heartbeat-doc accuracy, testable sequence seeding, logged failures, dead-code removal

src/ScadaLink.HealthMonitoring/CentralHealthReportLoop.cs

@@ -29,22 +29,31 @@ public class CentralHealthReportLoop : BackgroundService
 
     // Seeded with Unix-ms so reports from a newly-elected central leader
     // always sort after reports from any prior leader for siteId="central".
-    private long _sequenceNumber = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds();
+    // The clock is read through the injected TimeProvider so the seeding is
+    // deterministically testable.
+    private long _sequenceNumber;
 
     public CentralHealthReportLoop(
         ISiteHealthCollector collector,
         ICentralHealthAggregator aggregator,
         IClusterNodeProvider clusterNodeProvider,
         IOptions<HealthMonitoringOptions> options,
-        ILogger<CentralHealthReportLoop> logger)
+        ILogger<CentralHealthReportLoop> logger,
+        TimeProvider? timeProvider = null)
     {
         _collector = collector;
         _aggregator = aggregator;
         _clusterNodeProvider = clusterNodeProvider;
         _options = options.Value;
         _logger = logger;
+        _sequenceNumber = (timeProvider ?? TimeProvider.System).GetUtcNow().ToUnixTimeMilliseconds();
     }
 
+    /// <summary>
+    /// Current sequence number (for testing).
+    /// </summary>
+    public long CurrentSequenceNumber => Interlocked.Read(ref _sequenceNumber);
+
     protected override async Task ExecuteAsync(CancellationToken stoppingToken)
     {
         _logger.LogInformation(