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

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

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.S7/S7Driver.cs

@@ -32,7 +32,7 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.S7;
 ///     </para>
 /// </remarks>
 public sealed class S7Driver
-    : IDriver, ITagDiscovery, IReadable, IWritable, IBatchSubscribable, IHostConnectivityProbe, ISkippedTagReporter, IDisposable, IAsyncDisposable
+    : IDriver, ITagDiscovery, IReadable, IWritable, IBatchSubscribable, IHostConnectivityProbe, ISkippedTagReporter, IDriverDiagnostics, IDisposable, IAsyncDisposable
 {
     /// <summary>Authored tags this driver could not build into its poll table (Gitea #541).</summary>
     private readonly SkippedTagLedger _skippedTags = new();
@@ -463,6 +463,19 @@ public sealed class S7Driver
         ["S7.NegotiatedPduSize"] = NegotiatedPduSize,
     };
 
+    /// <summary>
+    ///     <see cref="IDriverDiagnostics"/> view of the same facts, as strings, for the <c>/hosts</c> page
+    ///     (Gitea #640). Empty before the first connect (PDU size 0 is "not negotiated yet", not a size).
+    /// </summary>
+    /// <returns>The diagnostics bag.</returns>
+    IReadOnlyDictionary<string, string> IDriverDiagnostics.GetDiagnostics()
+    {
+        var pdu = NegotiatedPduSize;
+        return pdu > 0
+            ? new Dictionary<string, string> { ["S7.NegotiatedPduSize"] = pdu.ToString(System.Globalization.CultureInfo.InvariantCulture) }
+            : new Dictionary<string, string>();
+    }
+
     private static readonly HashSet<S7DataType> ByteAnchoredDataTypes = new()
     {
         S7DataType.Int64,