test(audit): lock null-provider passthrough on CentralAuditWriter

Two follow-ups flagged by code review on Tasks 11/12:

- Lock the back-compat contract for CentralAuditWriter's optional
  `nodeIdentity = null` ctor parameter with two explicit tests
  (`WriteAsync_PassesThroughCallerSourceNode_WhenNoProviderInjected` and
  `WriteAsync_LeavesSourceNodeNull_WhenNoProviderInjected`). The previous
  null-provider path was only exercised incidentally via legacy
  CentralAuditWriterTests setups; the new tests make the contract explicit
  and distinct from the "provider supplied, returns null" path.

- Document why the catch-block log references `evt` rather than the
  post-stamp record: the three logged fields (EventId, Kind, Status) are
  immutable across the filter+stamp chain, so referencing either name is
  equivalent — but the comment warns future maintainers to switch names if
  they ever add a field the chain mutates (e.g. SourceNode).

tests/ScadaLink.AuditLog.Tests/Central/CentralAuditWriterTests.cs

@@ -180,4 +180,34 @@ public class CentralAuditWriterTests
             Arg.Is<AuditEvent>(e => e.SourceNode == null),
             Arg.Any<CancellationToken>());
     }
+
+    [Fact]
+    public async Task WriteAsync_PassesThroughCallerSourceNode_WhenNoProviderInjected()
+    {
+        // Locks the back-compat contract for the optional `nodeIdentity = null`
+        // ctor parameter: when no provider is wired (e.g. legacy M4 test
+        // composition roots), the writer must not stamp — caller value passes
+        // through unmodified. Distinct code path from
+        // "provider supplied, returns null", which the test above covers.
+        var (writer, repo) = BuildWriterWithIdentity(nodeIdentity: null);
+
+        await writer.WriteAsync(NewEvent() with { SourceNode = "node-z" });
+
+        await repo.Received(1).InsertIfNotExistsAsync(
+            Arg.Is<AuditEvent>(e => e.SourceNode == "node-z"),
+            Arg.Any<CancellationToken>());
+    }
+
+    [Fact]
+    public async Task WriteAsync_LeavesSourceNodeNull_WhenNoProviderInjected()
+    {
+        // Same back-compat contract for the null-caller-null-provider case.
+        var (writer, repo) = BuildWriterWithIdentity(nodeIdentity: null);
+
+        await writer.WriteAsync(NewEvent());
+
+        await repo.Received(1).InsertIfNotExistsAsync(
+            Arg.Is<AuditEvent>(e => e.SourceNode == null),
+            Arg.Any<CancellationToken>());
+    }
 }