docs(client): document Galaxy fallback operator-comment limitation + pin null behavior

Closes backlog #12: Galaxy's AckMsg attribute is WRITE-ONLY and the OPC UA
event SelectClause carries no comment field, making OperatorComment
unrecoverable on the sub-attribute fallback path. Documents the two concrete
reasons in-code and tightens the AlarmEventArgs XML doc; adds pinning test
OnAlarmEvent_GalaxyFallback_LeavesOperatorCommentNull.

src/Client/ZB.MOM.WW.OtOpcUa.Client.Shared/Models/AlarmEventArgs.cs

@@ -81,9 +81,13 @@ public sealed class AlarmEventArgs : EventArgs
 
     /// <summary>
     ///     PR E.7 — Operator-supplied comment recorded by the upstream alarm system on
-    ///     Acknowledge transitions. Null on raise / clear, or when the upstream path
-    ///     can't surface the comment (sub-attribute fallback path collapses comments
-    ///     into a single string write).
+    ///     Acknowledge transitions. Null on raise / clear events, and intentionally null
+    ///     on the Galaxy OPC UA sub-attribute fallback path for two unrecoverable reasons:
+    ///     (a) Galaxy's <c>AckMsg</c> attribute is WRITE-ONLY — no readable attribute
+    ///     exposes the last operator comment — and (b) the OPC UA event SelectClause
+    ///     carries no comment field. The operator comment is only available via the
+    ///     gateway's native alarm feed (<c>GalaxyAlarmTransition.OperatorComment</c>
+    ///     in the Galaxy driver path).
     /// </summary>
     public string? OperatorComment { get; }