docs(historian-gateway): correct the alarm-readback skip reason (SQL reader works)

Live investigation showed the earlier 'C2 server-gated event reads' attribution was
wrong: the gateway's SQL event reader works (a source-filtered ReadEvents returns a
real Galaxy-sourced event's history; a time-only ReadEvents returns 50 events). The
alarm round-trip's source-filtered readback is empty only because an ad-hoc SendEvent
is recorded in Runtime.dbo.Events WITHOUT a Source_Object — so reading existing Galaxy
alarm/event history by source works, but round-tripping OtOpcUa's own sends by source
needs the gateway's SendEvent to populate the event source. Skip message corrected.

Claude-Session: https://claude.ai/code/session_012SDSQ3AcaXqPcBtDESBRii

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Tests/Live/GatewayLiveIntegrationTests.cs

@@ -200,18 +200,20 @@ public sealed class GatewayLiveIntegrationTests(GatewayLiveFixture fixture) : IC
         }
         while (DateTime.UtcNow < deadline);
 
-        // The SendEvent itself is the OtOpcUa contract and is asserted above. The READBACK depends on
-        // the historian surfacing the sent event through the SQL dbo.Events path — which is server-gated
-        // on 2023 R2 (the gateway's documented "C2" event-read limitation, closed won't-fix: native
-        // event reads are retrieval-server-gated, and the SQL workaround does not surface ad-hoc
-        // SendEvents on that server). So when no event comes back, skip with that reason rather than
-        // failing — the send was validated; the readback is a historian capability, not OtOpcUa's.
+        // The SendEvent itself is the OtOpcUa contract and is asserted above. The source-filtered
+        // READBACK of a just-sent event is a historian/gateway property, not an OtOpcUa one: verified
+        // live, the gateway's SQL event reader works and source-filtering works for events that carry a
+        // Source_Object (real Galaxy-sourced events read back by source correctly), but an ad-hoc
+        // SendEvent is recorded in dbo.Events WITHOUT a Source_Object — so a source-filtered read of a
+        // freshly-sent event finds nothing. Reading existing Galaxy alarm/event history by source works;
+        // round-tripping OtOpcUa's OWN sends by source needs the gateway's SendEvent to populate the
+        // event source. So when no event comes back, skip with that reason rather than failing.
         if (events.Count == 0)
         {
             Assert.Skip(
-                $"Send acked, but ReadEvents returned 0 for source '{source}'. Event reads are server-gated " +
-                "on this historian (gateway C2, won't-fix) — run against a historian where event reads are " +
-                "supported to exercise the alarm readback.");
+                $"Send acked, but a source-filtered ReadEvents returned 0 for source '{source}'. The SQL event " +
+                "reader works (Galaxy-sourced events read back by source); an ad-hoc SendEvent is stored without " +
+                "a Source_Object, so its source-filtered readback is empty until the gateway populates the event source.");
         }
 
         var exactMatch = events.Any(e => string.Equals(e.EventId, alarmId, StringComparison.Ordinal));