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/DriverHostActorTests.cs

@@ -262,8 +262,25 @@ public sealed class DriverHostActorTests : RuntimeActorTestBase
             Status = status,
             CreatedBy = "test",
             SealedAtUtc = status == DeploymentStatus.Sealed ? DateTime.UtcNow : null,
+            // A REAL (if empty-of-content) artifact. These tests are about the apply/ACK/state machine,
+            // not about the artifact, and used to omit the blob entirely — but since #486 an unreadable
+            // artifact (which a missing blob is indistinguishable from) correctly FAILS the apply, so an
+            // artifact-less row would no longer exercise the success path they are asserting on. A
+            // configuration that declares no drivers is the genuine "nothing to run" deployment.
+            ArtifactBlob = EmptyButRealArtifact,
         });
         ctx.SaveChanges();
         return id;
     }
+
+    /// <summary>A well-formed artifact declaring no drivers — what the composer emits for a configuration
+    /// with nothing in it, as distinct from bytes that could not be read.</summary>
+    private static readonly byte[] EmptyButRealArtifact = JsonSerializer.SerializeToUtf8Bytes(new
+    {
+        RawFolders = Array.Empty<object>(),
+        DriverInstances = Array.Empty<object>(),
+        Devices = Array.Empty<object>(),
+        TagGroups = Array.Empty<object>(),
+        Tags = Array.Empty<object>(),
+    });
 }