docs(test): mark the audit builder-vs-wiring coverage boundary explicit

Code-review nit: Item C asserts BuildWriteFailureAuditEvent in isolation; the
single in-lock call-site wiring is covered by inspection + the production-proven
path (bb59fd4e). Documented as a deliberate boundary with a promote-if-second-
call-site note.

tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/NodeManagerWriteRevertTests.cs

@@ -23,6 +23,13 @@ namespace ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests;
 ///         the device Reason in its Message. (The end-to-end <c>Server.ReportEvent</c> dispatch needs a
 ///         subscribed event monitored-item to observe — the nearest deterministic seam is the builder.)</item>
 ///     </list>
+///     <para>
+///         Coverage boundary (deliberate): Item C asserts the audit-event <i>builder</i> in isolation,
+///         not its single call-site wiring inside <see cref="OtOpcUaNodeManager.RevertOptimisticWriteIfNeeded"/>
+///         (line ~1107). Observing that wiring end-to-end would require a subscribed event monitored-item;
+///         the one in-lock builder call is covered by inspection + the production-proven path (shipped
+///         <c>bb59fd4e</c>). If a second audit call-site is ever added, promote this to an observed-event test.
+///     </para>
 /// </summary>
 public sealed class NodeManagerWriteRevertTests : IDisposable
 {