feat(hosts): surface per-host connectivity on /hosts; drop the unwritable table (#521)

`IHostConnectivityProbe` was a dead surface: eleven drivers implement it, `GetHostStatuses()`
had ZERO production call sites, and `OnHostStatusChanged` had no subscriber outside the Galaxy
driver's own aggregator. Per-host connectivity was computed by every driver and read by nobody.

The issue offered "build the publisher or delete it". The publisher as its entity doc described
it — driver nodes upserting `DriverHostStatus` rows — is not buildable: per-cluster mesh Phase 4
gates `AddOtOpcUaConfigDb` on the `admin` role, so a driver-only node has no ConfigDb connection
to write rows with. So the capability is kept and the transport changed.

`DriverHealthChanged.HostStatuses` now carries the probe result to `/hosts` as a Hosts column.
That channel already reached the page, already survives the mesh split via the Phase 5 gRPC
telemetry stream, and already replays a last-value snapshot on re-subscribe — so per-host state
re-primes after a reconnect without a durable store. Both halves of the interface finally do
what they are for: the event triggers a prompt publish, the pull is the source of truth.

The point of the column is the case the driver-level state chip structurally cannot express: a
multi-device driver stays aggregate-Healthy while ONE of its devices is unreachable.

Two traps, both pinned by tests that were falsified against the prod code:

- The host digest MUST be in the publish fingerprint. On a single-host-down transition every
  other fingerprint component is unchanged, so the dedup would swallow exactly the publish
  carrying the news — the trap that already bit the rediscovery signal. Removing it turns the
  guard test red, verified.
- null (no probe) must stay distinct from empty (probe with no hosts). proto3 cannot tell an
  absent repeated field from an empty one, hence the explicit `has_host_statuses` flag;
  collapsing them would render every probe-less driver as one whose devices are all fine.

Dropped: the DriverHostStatus entity, enum, DbSet, model config and table (migration
DropDriverHostStatusTable — empty on every deployment, so the scaffolder's data-loss warning is
moot, and Down() recreates it exactly).

Found en route, NOT fixed here: `DriverInstanceResilienceStatus` is the identical defect — no
writer, no reader, only a DbSet declaration, while the live data rides the
`driver-resilience-status` telemetry channel. Its doc-comment now states that rather than
describing the sampler and AdminUI join that were never built. Filed as #524 rather than widening
this schema change beyond what was asked.

Claude-Session: https://claude.ai/code/session_015p7wGqy3YpZNCpDzTpGMKo

tests/Server/ZB.MOM.WW.OtOpcUa.Host.Tests/Grpc/TelemetryStreamGrpcServiceTests.cs

@@ -8,6 +8,10 @@ using ZB.MOM.WW.OtOpcUa.Commons.Messages.Logging;
 using ZB.MOM.WW.OtOpcUa.Commons.Protos.Telemetry.V1;
 using ZB.MOM.WW.OtOpcUa.Host.Grpc;
 using ZB.MOM.WW.OtOpcUa.Runtime.Telemetry;
+// Aliased, not imported wholesale: Core.Abstractions also declares a DriverHealth, which would collide
+// with the proto DriverHealth these tests assert on.
+using HostConnectivityStatus = ZB.MOM.WW.OtOpcUa.Core.Abstractions.HostConnectivityStatus;
+using HostState = ZB.MOM.WW.OtOpcUa.Core.Abstractions.HostState;
 
 namespace ZB.MOM.WW.OtOpcUa.Host.Tests.Grpc;
 
@@ -210,6 +214,40 @@ public sealed class TelemetryStreamGrpcServiceTests
         evt.DriverHealth.LastSuccessfulReadUtc.ToDateTime().ShouldBe(expectedUtc);
     }
 
+    /// <summary>
+    ///     Node side of the #521 host-status carry: the presence flag must be written, so the tri-state
+    ///     (no probe / probe with no hosts / probe with hosts) survives a wire proto3 cannot express on the
+    ///     repeated field alone. Paired with the central-side decode in <c>TelemetryProtoMapCentralTests</c>.
+    /// </summary>
+    [Fact]
+    public void ToProto_health_writes_the_host_status_presence_flag_and_entries()
+    {
+        var changedAt = new DateTime(2026, 7, 30, 11, 0, 0, DateTimeKind.Utc);
+        var withHosts = new DriverHealthChanged(
+            "cluster-a", "drv-1", "Healthy", null, null, 0, DateTime.UtcNow,
+            HostStatuses: [new HostConnectivityStatus("plc-a", HostState.Stopped, changedAt)]);
+
+        var evt = TelemetryProtoMapNode.ToProto(new TelemetryItem.Health(withHosts), "c");
+
+        evt.DriverHealth.HasHostStatuses.ShouldBeTrue();
+        evt.DriverHealth.HostStatuses.Count.ShouldBe(1);
+        evt.DriverHealth.HostStatuses[0].HostName.ShouldBe("plc-a");
+        evt.DriverHealth.HostStatuses[0].State.ShouldBe("Stopped");
+        evt.DriverHealth.HostStatuses[0].LastChangedUtc.ToDateTime().ShouldBe(changedAt);
+
+        // A probe reporting zero hosts still sets the flag — that is the whole point of having one.
+        var emptyProbe = new DriverHealthChanged(
+            "cluster-a", "drv-1", "Healthy", null, null, 0, DateTime.UtcNow, HostStatuses: []);
+        var emptyEvt = TelemetryProtoMapNode.ToProto(new TelemetryItem.Health(emptyProbe), "c");
+        emptyEvt.DriverHealth.HasHostStatuses.ShouldBeTrue();
+        emptyEvt.DriverHealth.HostStatuses.ShouldBeEmpty();
+
+        // No probe: flag clear.
+        var noProbe = new DriverHealthChanged("cluster-a", "drv-1", "Healthy", null, null, 0, DateTime.UtcNow);
+        TelemetryProtoMapNode.ToProto(new TelemetryItem.Health(noProbe), "c")
+            .DriverHealth.HasHostStatuses.ShouldBeFalse();
+    }
+
     [Fact]
     public async Task Client_disconnect_mid_stream_ends_cleanly_without_leaking_a_slot()
     {