fix(opcuaclient): review fixes - drop driver-side OperationLimits chunking (SDK batches), diagnostics as OTel counter, safe discovery selection, options validation at load (#273 #275 #276 #277 #278)

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

src/Core/ZB.MOM.WW.OtOpcUa.Commons/Observability/OtOpcUaTelemetry.cs

@@ -76,6 +76,16 @@ public static class OtOpcUaTelemetry
     /// </summary>
     public const string PollOverrunDriverInstanceTag = "driver_instance";
 
+    /// <summary>
+    /// Session/publish-level events of an OPC UA Client driver instance (#276): the operator's view of a
+    /// flapping upstream. Tagged <see cref="PollOverrunDriverInstanceTag"/> (<c>driver_instance</c>) +
+    /// <c>event</c> = <c>publish_response|notification|data_change|bad_status_value|publish_error|
+    /// keepalive_miss|reconnect_attempt|reconnect_success|reconnect_exhausted</c>.
+    /// </summary>
+    public static readonly Counter<long> OpcUaClientEvents =
+        Meter.CreateCounter<long>("otopcua.driver.opcuaclient.events", unit: "{event}",
+            description: "OPC UA Client driver session/publish events (event=publish_response|notification|data_change|bad_status_value|publish_error|keepalive_miss|reconnect_attempt|reconnect_success|reconnect_exhausted).");
+
     // ---------------- VirtualTag / ScriptedAlarm engines ----------------
 
     public static readonly Counter<long> VirtualTagEval =

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

@@ -15,16 +15,10 @@ namespace ZB.MOM.WW.OtOpcUa.Core.Abstractions;
 ///     carry a non-null message. Callers must not key behaviour on the LastError-null ↔ Healthy
 ///     pairing.
 /// </param>
-/// <param name="Diagnostics">
-///     Optional driver-specific counters/gauges (name → value) for the diagnostics surface (#276).
-///     Null for every driver that publishes none. Deliberately NOT part of any health-change
-///     fingerprint: counters move on every poll, so keying a dedup on them would defeat it.
-/// </param>
 public sealed record DriverHealth(
     DriverState State,
     DateTime? LastSuccessfulRead,
-    string? LastError,
-    IReadOnlyDictionary<string, double>? Diagnostics = null);
+    string? LastError);
 
 /// <summary>Driver-instance lifecycle state.</summary>
 public enum DriverState