fix(drivers): fail the apply when the artifact could not be read (#486)

ApplyAndAck advanced _currentRevision and recorded NodeDeploymentState=Applied
immediately after ReconcileDrivers, which returns null when the artifact could
not be read. So a node that applied NOTHING reported success, claimed a
revision whose configuration it never applied, and — because
HandleDispatchFromSteady short-circuits on a revision match — could never be
healed by re-dispatching that revision. Only a later, different revision
recovered it.

Now a null blob fails the apply: the revision is left where it was,
NodeDeploymentState records Failed with the reason, and the coordinator gets a
Failed ACK. Leaving the revision alone is what makes the retry actually land
instead of being waved through.

This is about telling the truth, not about tearing anything down — the node
keeps serving its last-known-good address space, drivers and subscriptions
throughout (#485).

Tests, RED-first (both verified failing with "should be Failed but was
Applied"): the ACK/revision assertion, plus the one that matters — after a
failed apply, re-dispatching the SAME revision now genuinely applies. Its
recovered artifact adds a second driver precisely so a short-circuited ACK
(which has no side effects) cannot satisfy it.

Four existing tests changed, and both changes are deliberate:

- DriverHostActorTests.SeedDeployment never set ArtifactBlob at all. Its three
  consumers are about the apply/ACK/state machine, not the artifact, and now
  need a genuine apply — so the helper seeds a well-formed artifact declaring
  no drivers. That is what the composer emits for an empty configuration, and
  is precisely what "bytes we could not read" is NOT.
- EmptyArtifact_IsNotCached asserted "the apply still succeeds — an empty
  artifact is a legitimate no-op deployment". That premise is the bug. Flipped
  to Failed; the test's actual subject (nothing is cached) is untouched and now
  holds for two independent reasons.

Closes #486

Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW

tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Drivers/DriverHostActorArtifactCacheTests.cs

@@ -129,10 +129,12 @@ public sealed class DriverHostActorArtifactCacheTests : RuntimeActorTestBase
 
         actor.Tell(new DispatchDeployment(deploymentId, RevA, CorrelationId.NewId()));
 
-        // The apply still succeeds — an empty artifact is a legitimate no-op deployment.
-        coordinator.ExpectMsg<ApplyAck>(TimeSpan.FromSeconds(5)).Outcome.ShouldBe(ApplyAckOutcome.Applied);
+        // Since #486 the apply FAILS rather than reporting a success it did not achieve: empty bytes are
+        // what an unreadable artifact looks like, not a legitimate no-op deployment. (A configuration that
+        // genuinely declares nothing still serialises to a JSON document with empty arrays.)
+        coordinator.ExpectMsg<ApplyAck>(TimeSpan.FromSeconds(5)).Outcome.ShouldBe(ApplyAckOutcome.Failed);
 
-        // But nothing is cached from it.
+        // The point of this test is unchanged, and now holds for two independent reasons: nothing is cached.
         Thread.Sleep(500);
         cache.Stores.ShouldBeEmpty();
     }