fix(high-severity): close 9 of 10 open High findings across 8 modules

Comm-016: delete dead HandleConnectionStateChanged + _debugSubscriptions /
_inProgressDeployments tracking + ConnectionStateChanged message record.
Disconnect detection is owned by the transport layers (gRPC keepalive PING
~25s; Ask-timeout at CommunicationService). Updates the
Component-Communication.md design doc to make that explicit.

SnF-018: NotificationForwarder.DeliverAsync now discards a corrupt buffered
payload (Warning log + return true) instead of returning false and parking
the row — honoring the design's "notifications do not park" invariant.

DM-018: reconciliation no longer force-sets Enabled, preserving an
intentional Disabled state after central failover.

ESG-018: DeliverBufferedAsync (both ExternalSystemClient + DatabaseGateway)
catches JsonException and returns false, turning a corrupt buffered row
into a parked operation instead of a retry-forever poison message.

InboundAPI-022: register ActiveNodeGate as IActiveNodeGate in the Central
DI branch so standby-node gating is actually wired up in production.

NS-019: remove orphaned NotificationDeliveryService /
INotificationDeliveryService / NotificationResult; central notification
delivery now lives entirely in NotificationOutbox.

SEL-016: normalise From/To filters to UTC before ISO-string compare so
non-UTC DateTimeOffset clients no longer get spuriously excluded events.

TE-017: include Description on attributes/alarms and a HashableConnections
projection (protocol, endpoint JSON, failover count) in the revision hash
and DiffService; staleness detection now catches description-only and
connection-endpoint edits.

Transport-001 and Transport-002 (also High) remain Open — they're being
handled in a follow-up batch because both touch BundleImporter.cs and
must serialise.

tests/ScadaLink.ExternalSystemGateway.Tests/ExternalSystemClientTests.cs

@@ -234,6 +234,32 @@ public class ExternalSystemClientTests
             () => client.DeliverBufferedAsync(BufferedCall("TestAPI", "failMethod")));
     }
 
+    // ── ExternalSystemGateway-018: malformed JSON payload must park, not retry-forever ──
+
+    [Fact]
+    public async Task DeliverBuffered_MalformedJsonPayload_ReturnsFalseSoMessageParks()
+    {
+        // No repository / HTTP stubs needed — deserialization fails before any
+        // resolution or HTTP call. If the JsonException were to escape (the
+        // pre-018 behaviour) the S&F engine would treat it as transient and
+        // retry the same poison row forever.
+        var client = new ExternalSystemClient(
+            _httpClientFactory, _repository, NullLogger<ExternalSystemClient>.Instance);
+
+        var poisonMessage = new StoreAndForwardMessage
+        {
+            Id = Guid.NewGuid().ToString("N"),
+            Category = ScadaLink.Commons.Types.Enums.StoreAndForwardCategory.ExternalSystem,
+            Target = "TestAPI",
+            // Truncated mid-write — `{` opens an object that never closes.
+            PayloadJson = "{\"SystemName\":\"TestAPI\",\"MethodName\":\"get",
+        };
+
+        var delivered = await client.DeliverBufferedAsync(poisonMessage);
+
+        Assert.False(delivered); // permanent — the S&F engine parks the message
+    }
+
     // ── ExternalSystemGateway-003: CachedCall must not double-dispatch ──
 
     [Fact]