fix(galaxy): invalidate writer handle caches on session reconnect

Add IGalaxyDataWriter.InvalidateHandleCaches() and call it in
GalaxyDriver.ReopenAsync after RecreateAsync succeeds. Prior to this
fix, GatewayGalaxyDataWriter's _itemHandles and _supervisedHandles
dictionaries survived across reconnects, causing the next write to
skip AddItem and AdviseSupervisory against already-dead handles.

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/GalaxyDriver.cs

@@ -293,6 +293,9 @@ public sealed class GalaxyDriver
         if (_ownedMxSession is null) return;
         var clientOptions = BuildClientOptions(_options.Gateway);
         await _ownedMxSession.RecreateAsync(clientOptions, cancellationToken).ConfigureAwait(false);
+        // The recreated session invalidates every prior gw item handle; drop the writer's handle/advise
+        // caches so the next write re-AddItems + re-AdviseSupervisory against the fresh session.
+        _dataWriter?.InvalidateHandleCaches();
     }
 
     /// <summary>

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Runtime/GatewayGalaxyDataWriter.cs

@@ -42,6 +42,42 @@ public sealed class GatewayGalaxyDataWriter : IGalaxyDataWriter
         _logger = logger ?? NullLogger.Instance;
     }
 
+    /// <inheritdoc />
+    public void InvalidateHandleCaches()
+    {
+        _itemHandles.Clear();
+        _supervisedHandles.Clear();
+    }
+
+    // ===== Test seams (internal — not part of the public contract) =====
+
+    /// <summary>
+    ///     Count of item-handle cache entries. Zero on a fresh instance or immediately after
+    ///     <see cref="InvalidateHandleCaches"/>. Used by unit tests to verify cache state
+    ///     without running a real gRPC round-trip.
+    /// </summary>
+    internal int CachedItemHandleCount => _itemHandles.Count;
+
+    /// <summary>
+    ///     Count of supervisory-advised handle entries. Zero on a fresh instance or immediately
+    ///     after <see cref="InvalidateHandleCaches"/>. Used by unit tests to verify cache state.
+    /// </summary>
+    internal int CachedSupervisedHandleCount => _supervisedHandles.Count;
+
+    /// <summary>
+    ///     Pre-populate both caches as if a write had already occurred. Used by unit tests to
+    ///     simulate the "post-write" state without running a real gRPC gateway session (the SDK
+    ///     session types are sealed + internal-ctor and cannot be faked).
+    /// </summary>
+    /// <param name="fullRef">The tag full reference to add to the item-handle cache.</param>
+    /// <param name="itemHandle">The item handle to cache for that reference.</param>
+    /// <param name="supervised">When true, also records the handle in the supervised-handle cache.</param>
+    internal void SeedHandleCachesForTest(string fullRef, int itemHandle, bool supervised)
+    {
+        _itemHandles[fullRef] = itemHandle;
+        if (supervised) _supervisedHandles.TryAdd(itemHandle, 0);
+    }
+
     /// <summary>Writes values to Galaxy tags through the gateway.</summary>
     /// <param name="writes">The write requests.</param>
     /// <param name="securityResolver">Function to resolve security classification per tag.</param>

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Runtime/IGalaxyDataWriter.cs

@@ -30,4 +30,8 @@ public interface IGalaxyDataWriter
         IReadOnlyList<WriteRequest> writes,
         Func<string, SecurityClassification> securityResolver,
         CancellationToken cancellationToken);
+
+    /// <summary>Drop cached gateway item handles + supervisory-advise state. Call after a session
+    /// reconnect — the prior handles are dead, so the next write must re-AddItem + re-AdviseSupervisory.</summary>
+    void InvalidateHandleCaches();
 }

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Runtime/TracedGalaxyDataWriter.cs

@@ -10,6 +10,10 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Runtime;
 /// </summary>
 internal sealed class TracedGalaxyDataWriter(IGalaxyDataWriter inner, string clientName) : IGalaxyDataWriter
 {
+    /// <inheritdoc />
+    /// <remarks>No span — this is a local cache-clear operation, not a gateway round-trip.</remarks>
+    public void InvalidateHandleCaches() => inner.InvalidateHandleCaches();
+
     /// <summary>Writes data to Galaxy while recording telemetry span.</summary>
     /// <param name="writes">The list of write requests to process.</param>
     /// <param name="securityResolver">Function to resolve security classification for tag references.</param>