test(sitecall-audit): symmetric SourceNode coverage on DbOutbound emitter + clarify DI comments

Two follow-ups from the T13/T14 code review:

- M1: Add CachedWrite_StampsSourceNode_OnSubmitTelemetryRow and
  CachedWrite_NoSourceNodeWired_LeavesSourceNodeNull to DatabaseCachedWriteEmissionTests,
  mirroring the existing ApiOutbound SourceNode tests in
  ExternalSystemCachedCallEmissionTests. Site-emitter coverage now symmetric
  across both cached-call channels.
- M2: Clarify the GetService(INodeIdentityProvider) DI comments on the
  CachedCallTelemetryForwarder and CachedCallLifecycleBridge factories:
  it's test composition roots that may not register the provider, not
  central production. Both site and central hosts always register it via
  SiteServiceRegistration.BindSharedOptions.

src/ScadaLink.AuditLog/ServiceCollectionExtensions.cs

@@ -149,10 +149,13 @@ public static class ServiceCollectionExtensions
                 sp.GetRequiredService<ILogger<CachedCallTelemetryForwarder>>(),
                 // SourceNode-stamping (Task 14): the local node identity is
                 // threaded through so RecordEnqueueAsync can stamp the
-                // tracking row's SourceNode column. GetService — central
-                // composition roots may not register the provider, in which
-                // case the forwarder degrades to a null SourceNode rather
-                // than failing the DI resolution.
+                // tracking row's SourceNode column. GetService (not
+                // GetRequiredService) — test composition roots that build a
+                // stripped DI container may not register the provider, in
+                // which case the forwarder degrades to a null SourceNode
+                // rather than failing the DI resolution. Production hosts
+                // (site + central) always register it via
+                // SiteServiceRegistration.BindSharedOptions.
                 sp.GetService<INodeIdentityProvider>()));
 
         // M3 Bundle F: bridge the store-and-forward retry-loop observer hook
@@ -165,9 +168,11 @@ public static class ServiceCollectionExtensions
         // INodeIdentityProvider singleton can be threaded through — the
         // bridge stamps SiteCallOperational.SourceNode from
         // INodeIdentityProvider.NodeName on every cached-call lifecycle row.
-        // GetService (not GetRequiredService) — central composition roots may
-        // not register the provider, in which case the bridge degrades to a
-        // null SourceNode rather than failing the DI resolution.
+        // GetService (not GetRequiredService) — test composition roots that
+        // build a stripped DI container may not register the provider, in
+        // which case the bridge degrades to a null SourceNode rather than
+        // failing the DI resolution. Production hosts (site + central)
+        // always register it via SiteServiceRegistration.BindSharedOptions.
         services.AddSingleton<CachedCallLifecycleBridge>(sp => new CachedCallLifecycleBridge(
             sp.GetRequiredService<ICachedCallTelemetryForwarder>(),
             sp.GetRequiredService<ILogger<CachedCallLifecycleBridge>>(),