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).

src/ScadaLink.AuditLog/Central/CentralAuditWriter.cs

@@ -136,6 +136,13 @@ public sealed class CentralAuditWriter : ICentralAuditWriter
                 // misbehaving custom counter does, swallowing here keeps the
                 // best-effort contract intact.
             }
+            // Log the input event's identifying fields. These three (EventId,
+            // Kind, Status) are immutable across the filter+stamp chain — the
+            // `with` clones above touch only SourceNode and IngestedAtUtc — so
+            // referencing `evt` here is intentional and equivalent to the
+            // stamped record for diagnostics. If you add a field here that the
+            // stamp chain DOES mutate (e.g., SourceNode), reference the latest
+            // post-stamp record name instead, not `evt`.
             _logger.LogWarning(
                 ex,
                 "CentralAuditWriter failed for EventId {EventId} (Kind={Kind}, Status={Status})",