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.Commons/Interfaces/Transport/IBundleSessionStore.cs

@@ -18,6 +18,15 @@ public interface IBundleSessionStore
     /// <summary>Removes all sessions whose expiry has passed.</summary>
     void EvictExpired();
 
+    /// <summary>
+    /// Register row 53: the number of sessions currently held that have not yet expired.
+    /// Backs the <c>scadabridge.central.transport.import_sessions.open</c> gauge, so it
+    /// must be cheap and side-effect-free (it is read from the metrics collection thread)
+    /// and must NOT count expired-but-unswept entries — otherwise the gauge would read
+    /// as "at the cap" for up to a sweep interval after a burst drained.
+    /// </summary>
+    int OpenSessionCount { get; }
+
     /// <summary>
     /// T-003: returns the current unlock-failure count for a bundle keyed by its
     /// content hash. The counter is server-owned so a second tab / CLI caller