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

src/Server/ZB.MOM.WW.OtOpcUa.ControlPlane/Telemetry/TelemetryProtoMapCentral.cs

@@ -3,6 +3,10 @@ using ZB.MOM.WW.OtOpcUa.Commons.Messages.Alerts;
 using ZB.MOM.WW.OtOpcUa.Commons.Messages.Drivers;
 using ZB.MOM.WW.OtOpcUa.Commons.Messages.Logging;
 using ZB.MOM.WW.OtOpcUa.Commons.Protos.Telemetry.V1;
+// Aliased, not imported wholesale: Core.Abstractions also declares a DriverHealth, which would collide
+// with the proto DriverHealth this file maps.
+using HostConnectivityStatus = ZB.MOM.WW.OtOpcUa.Core.Abstractions.HostConnectivityStatus;
+using HostState = ZB.MOM.WW.OtOpcUa.Core.Abstractions.HostState;
 
 namespace ZB.MOM.WW.OtOpcUa.ControlPlane.Telemetry;
 
@@ -120,7 +124,32 @@ public static class TelemetryProtoMapCentral
             ErrorCount5Min: msg.ErrorCount5Min,
             PublishedUtc: Required(msg.PublishedUtc, "DriverHealth", "published_utc"),
             RediscoveryNeededUtc: msg.RediscoveryNeededUtc?.ToDateTime(),
-            RediscoveryReason: msg.HasRediscoveryReason ? msg.RediscoveryReason : null);
+            RediscoveryReason: msg.HasRediscoveryReason ? msg.RediscoveryReason : null,
+            HostStatuses: ToHostStatuses(msg));
+    }
+
+    /// <summary>
+    ///     Projects the repeated host-connectivity field, honouring the explicit presence flag: null when
+    ///     the node said the driver has no probe, an empty list when it has one that knows no hosts.
+    ///     <para>An unparseable state string degrades to <see cref="HostState.Unknown"/> rather than
+    ///     throwing — a node running a newer build that added an enum member must not be able to kill
+    ///     central's telemetry stream, which is observability and has no business failing closed.</para>
+    /// </summary>
+    private static IReadOnlyList<HostConnectivityStatus>? ToHostStatuses(DriverHealth msg)
+    {
+        if (!msg.HasHostStatuses) return null;
+
+        var result = new List<HostConnectivityStatus>(msg.HostStatuses.Count);
+        foreach (var h in msg.HostStatuses)
+        {
+            result.Add(new HostConnectivityStatus(
+                h.HostName,
+                // System.Enum qualified: Google.Protobuf.WellKnownTypes also declares an Enum type.
+                System.Enum.TryParse<HostState>(h.State, ignoreCase: true, out var state) ? state : HostState.Unknown,
+                h.LastChangedUtc?.ToDateTime() ?? default));
+        }
+
+        return result;
     }
 
     /// <summary>Projects a <see cref="DriverResilienceStatus"/> onto a <see cref="DriverResilienceStatusChanged"/>.</summary>