feat(transport): observability for the import-session cap (register row 53)

Nothing counted open import sessions and nothing logged a rejection at
MaxConcurrentImportSessions (default 8), so the register-row-43 session leak
could only be inferred from rotating test failures.

- Gauge scadabridge.central.transport.import_sessions.open on the existing
  ZB.MOM.WW.ScadaBridge meter — already in SiteServiceRegistration.ObservedMeters
  (the ZbTelemetryOptions.Meters allowlist), so no telemetry config change is
  needed and the series is not silently dropped. Provider-backed, mirroring the
  StoreAndForward queue-depth gauge: registered by BundleSessionEvictionService
  .StartAsync (Transport's only lifecycle seam) and identity-checked-cleared on
  StopAsync so a late stop cannot clobber a newer registrant.
- IBundleSessionStore.OpenSessionCount counts only UNEXPIRED sessions. The raw
  dictionary count would report the node as pinned at the cap for up to a
  one-minute sweep interval after a burst had drained — the exact false positive
  the instrument exists to rule out.
- BundleSessionStore.Open logs Warning with open count, cap and TTL when it
  rejects. Fires only on a real cap hit: a store full of expired sessions is
  reclaimed first and logs nothing.
- Docs: docs/components/Transport.md + docs/requirements/Component-Transport.md.

Tests: Transport 173 passed (+6 new — Warning content, two negative controls,
unexpired-only counting, gauge register/clear, no-clobber-on-late-stop).

src/ZB.MOM.WW.ScadaBridge.Transport/Import/BundleSessionEvictionService.cs

@@ -1,6 +1,7 @@
 using Microsoft.Extensions.Hosting;
 using Microsoft.Extensions.Logging;
 using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Transport;
+using ZB.MOM.WW.ScadaBridge.Commons.Observability;
 
 namespace ZB.MOM.WW.ScadaBridge.Transport.Import;
 
@@ -21,6 +22,15 @@ internal sealed class BundleSessionEvictionService : BackgroundService
     private readonly IBundleSessionStore _sessionStore;
     private readonly ILogger<BundleSessionEvictionService> _logger;
 
+    /// <summary>
+    /// Register row 53: the open-session gauge callback, held in a field so
+    /// <see cref="StopAsync"/> can pass the SAME delegate reference to
+    /// <see cref="ScadaBridgeTelemetry.ClearImportSessionCountProvider"/> — the clear is
+    /// an identity-checked compare-and-clear, so a stale stop cannot unregister a newer
+    /// instance's provider. Mirrors StoreAndForwardService's queue-depth registration.
+    /// </summary>
+    private readonly Func<long> _openSessionCountProvider;
+
     /// <summary>
     /// Initializes a new instance of <see cref="BundleSessionEvictionService"/>.
     /// </summary>
@@ -32,6 +42,35 @@ internal sealed class BundleSessionEvictionService : BackgroundService
     {
         _sessionStore = sessionStore ?? throw new ArgumentNullException(nameof(sessionStore));
         _logger = logger ?? throw new ArgumentNullException(nameof(logger));
+        _openSessionCountProvider = () => _sessionStore.OpenSessionCount;
+    }
+
+    /// <summary>
+    /// Register row 53: registers the open-import-sessions gauge provider before the
+    /// sweep loop starts. This service is the Transport component's only lifecycle seam
+    /// (the session store itself is a plain singleton), and it is registered by
+    /// <c>AddTransport</c>, which only the Central branch of the host calls — so the
+    /// gauge reports 0 on a site node, where there are no import sessions to count.
+    /// </summary>
+    /// <param name="cancellationToken">Host startup cancellation token.</param>
+    /// <returns>A task that completes when the base <see cref="BackgroundService"/> has started.</returns>
+    public override Task StartAsync(CancellationToken cancellationToken)
+    {
+        ScadaBridgeTelemetry.SetImportSessionCountProvider(_openSessionCountProvider);
+        return base.StartAsync(cancellationToken);
+    }
+
+    /// <summary>
+    /// Deregisters this instance's gauge provider on graceful stop so the gauge falls
+    /// back to 0 rather than reading through a dead store. The clear is identity-checked,
+    /// so a late stop cannot clobber a successor that already re-registered.
+    /// </summary>
+    /// <param name="cancellationToken">Host shutdown cancellation token.</param>
+    /// <returns>A task that completes when the base <see cref="BackgroundService"/> has stopped.</returns>
+    public override async Task StopAsync(CancellationToken cancellationToken)
+    {
+        ScadaBridgeTelemetry.ClearImportSessionCountProvider(_openSessionCountProvider);
+        await base.StopAsync(cancellationToken).ConfigureAwait(false);
     }
 
     /// <inheritdoc />