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.Core.Abstractions/IDriverDiagnostics.cs

@@ -0,0 +1,58 @@
+namespace ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+/// <summary>
+///     Optional driver capability: expose a small bag of operator-facing diagnostic key/value pairs
+///     (e.g. S7's negotiated PDU size) that ride the driver-health snapshot to the AdminUI <c>/hosts</c>
+///     page (Gitea #640).
+/// </summary>
+/// <remarks>
+///     <para>
+///         <b>Why a bag rather than fields on <see cref="DriverHealth"/>.</b> The facts worth showing are
+///         driver-specific and do not change the driver's <i>state</i>; a typed field per driver would
+///         widen the core record and the telemetry proto for every driver. Same additive-never-a-state-change
+///         rule as <see cref="ISkippedTagReporter"/>.
+///     </para>
+///     <para>
+///         Values are strings so the telemetry proto stays a plain <c>map&lt;string,string&gt;</c>. Keep the
+///         bag small (<see cref="DriverDiagnostics.MaxEntries"/> entries, text truncated to
+///         <see cref="DriverDiagnostics.MaxValueLength"/>): it is published on every state change.
+///     </para>
+/// </remarks>
+public interface IDriverDiagnostics
+{
+    /// <summary>
+    ///     Snapshot of the driver's diagnostics. Must be a pure in-memory read with no I/O — it is called on
+    ///     the health-publish path, deliberately NOT through the resilience pipeline.
+    /// </summary>
+    /// <returns>The key/value pairs; empty when there is nothing to show yet.</returns>
+    IReadOnlyDictionary<string, string> GetDiagnostics();
+}
+
+/// <summary>Bounds and helpers shared by every consumer of <see cref="IDriverDiagnostics"/>.</summary>
+public static class DriverDiagnostics
+{
+    /// <summary>Maximum number of entries carried on a health snapshot; extras are dropped (ordinal key order).</summary>
+    public const int MaxEntries = 16;
+
+    /// <summary>Maximum length of a key or value carried on a health snapshot; longer text is truncated.</summary>
+    public const int MaxValueLength = 200;
+
+    /// <summary>
+    ///     Normalises a driver's bag for publishing: ordinal key order, capped entry count, truncated text.
+    ///     Deterministic so the same facts always produce the same fingerprint digest.
+    /// </summary>
+    /// <param name="raw">The bag the driver returned.</param>
+    /// <returns>A bounded, ordered copy.</returns>
+    public static IReadOnlyDictionary<string, string> Bound(IReadOnlyDictionary<string, string> raw)
+    {
+        var result = new SortedDictionary<string, string>(StringComparer.Ordinal);
+        foreach (var kv in raw.OrderBy(k => k.Key, StringComparer.Ordinal))
+        {
+            if (result.Count >= MaxEntries) break;
+            result[Truncate(kv.Key ?? "")] = Truncate(kv.Value ?? "");
+        }
+        return result;
+    }
+
+    private static string Truncate(string s) => s.Length <= MaxValueLength ? s : s[..MaxValueLength];
+}

src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/IDriverHealthPublisher.cs

@@ -34,6 +34,10 @@ public interface IDriverHealthPublisher
     ///     <see cref="ISkippedTagReporter"/>. Contradicts the "Healthy, 0 errors, reads advancing" reading
     ///     of a driver that is in fact ignoring part or all of its configuration (Gitea #541).
     /// </param>
+    /// <param name="diagnostics">
+    ///     Bounded driver diagnostics from <see cref="IDriverDiagnostics.GetDiagnostics"/>; null when the
+    ///     driver is not an <see cref="IDriverDiagnostics"/> (Gitea #640).
+    /// </param>
     void Publish(
         string clusterId,
         string driverInstanceId,
@@ -42,7 +46,8 @@ public interface IDriverHealthPublisher
         DateTime? rediscoveryNeededUtc = null,
         string? rediscoveryReason = null,
         IReadOnlyList<HostConnectivityStatus>? hostStatuses = null,
-        SkippedTagReport? skippedTags = null);
+        SkippedTagReport? skippedTags = null,
+        IReadOnlyDictionary<string, string>? diagnostics = null);
 }
 
 /// <summary>
@@ -65,6 +70,7 @@ public sealed class NullDriverHealthPublisher : IDriverHealthPublisher
         DateTime? rediscoveryNeededUtc = null,
         string? rediscoveryReason = null,
         IReadOnlyList<HostConnectivityStatus>? hostStatuses = null,
-        SkippedTagReport? skippedTags = null)
+        SkippedTagReport? skippedTags = null,
+        IReadOnlyDictionary<string, string>? diagnostics = null)
     { /* no-op */ }
 }