feat(hosts): driver diagnostics channel on /hosts, S7 negotiated PDU size first (#640)

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe

tests/Server/ZB.MOM.WW.OtOpcUa.ControlPlane.Tests/Telemetry/TelemetryProtoMapCentralTests.cs

@@ -276,6 +276,43 @@ public sealed class TelemetryProtoMapCentralTests
         TelemetryProtoMapCentral.ToHealth(absentSkips).SkippedTags.ShouldBeNull();
     }
 
+    /// <summary>
+    ///     #640: the diagnostics bag round-trips with the null vs empty distinction intact, and a pre-#640
+    ///     node (field never set) decodes to null.
+    /// </summary>
+    [Fact]
+    public void ToHealth_diagnostics_round_trip_with_the_null_vs_empty_distinction_intact()
+    {
+        var populated = new DriverHealth
+        {
+            ClusterId = "c1", DriverInstanceId = "d1", State = "Healthy",
+            PublishedUtc = Timestamp.FromDateTime(SampleUtc),
+            HasDiagnostics = true,
+            Diagnostics = { ["S7.NegotiatedPduSize"] = "480", ["Other"] = "x" },
+        };
+        var mapped = TelemetryProtoMapCentral.ToHealth(populated).Diagnostics;
+        mapped.ShouldNotBeNull();
+        mapped!.Count.ShouldBe(2);
+        mapped["S7.NegotiatedPduSize"].ShouldBe("480");
+
+        var empty = new DriverHealth
+        {
+            ClusterId = "c1", DriverInstanceId = "d1", State = "Healthy",
+            PublishedUtc = Timestamp.FromDateTime(SampleUtc),
+            HasDiagnostics = true,
+        };
+        var mappedEmpty = TelemetryProtoMapCentral.ToHealth(empty).Diagnostics;
+        mappedEmpty.ShouldNotBeNull();
+        mappedEmpty!.ShouldBeEmpty();
+
+        var absent = new DriverHealth
+        {
+            ClusterId = "c1", DriverInstanceId = "d1", State = "Healthy",
+            PublishedUtc = Timestamp.FromDateTime(SampleUtc),
+        };
+        TelemetryProtoMapCentral.ToHealth(absent).Diagnostics.ShouldBeNull();
+    }
+
     /// <summary>
     ///     An unparseable host state 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