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

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

src/Core/ZB.MOM.WW.OtOpcUa.Commons/Messages/Drivers/DriverHealthChanged.cs

@@ -59,6 +59,12 @@ namespace ZB.MOM.WW.OtOpcUa.Commons.Messages.Drivers;
 ///     <para><b>Value-equality caveat</b> — the same one <paramref name="HostStatuses"/> carries: the
 ///     record's generated <c>Equals</c> compares the sample list by REFERENCE.</para>
 /// </param>
+/// <param name="Diagnostics">
+///     Operator-facing driver diagnostics from <c>IDriverDiagnostics.GetDiagnostics()</c> (e.g. S7's
+///     negotiated PDU size), bounded and in ordinal key order; null when the driver does not implement the
+///     capability (Gitea #640). Additive, never a state change. Same by-reference value-equality caveat as
+///     <paramref name="HostStatuses"/>.
+/// </param>
 public sealed record DriverHealthChanged(
     string ClusterId,
     string DriverInstanceId,
@@ -70,7 +76,8 @@ public sealed record DriverHealthChanged(
     DateTime? RediscoveryNeededUtc = null,
     string? RediscoveryReason = null,
     IReadOnlyList<HostConnectivityStatus>? HostStatuses = null,
-    SkippedTagReport? SkippedTags = null)
+    SkippedTagReport? SkippedTags = null,
+    IReadOnlyDictionary<string, string>? Diagnostics = null)
 {
     /// <summary>
     ///     DPS topic name. Both the runtime <c>AkkaDriverHealthPublisher</c> and the AdminUI

src/Core/ZB.MOM.WW.OtOpcUa.Commons/Protos/telemetry.proto

@@ -82,6 +82,12 @@ message DriverHealth {
   // proto3, which a repeated field does not. So no companion has_ flag (and one would collide: protoc
   // already generates a HasSkippedTags property for this field).
   SkippedTags skipped_tags = 12;
+  // Driver diagnostics bag (Gitea #640), e.g. S7's negotiated PDU size. Additive: an old node never sets
+  // these and an old central ignores them. A map cannot distinguish absent from empty, and the two differ
+  // ("driver has no IDriverDiagnostics" vs "it has one and currently has nothing to say"), so the presence
+  // flag carries that bit - the same shape as has_host_statuses.
+  bool has_diagnostics = 13;
+  map<string, string> diagnostics = 14;
 }
 
 // Mirrors ZB.MOM.WW.OtOpcUa.Core.Abstractions.SkippedTagReport.