fix(client-shared): resolve High code-review findings (Client.Shared-005, Client.Shared-006)

Client.Shared-005: _activeDataSubscriptions (a plain Dictionary) and the
_activeAlarmSubscription tuple were mutated from the caller thread, the
keep-alive failover path, and DisconnectAsync with no synchronization,
risking bucket corrosion / InvalidOperationException / lost entries.
Added a dedicated _subscriptionLock and wrapped every read/write of that
bookkeeping state inside it (Subscribe/Unsubscribe[Alarms]Async,
Disconnect, Dispose, and the snapshot/clear/re-record steps of
ReplaySubscriptionsAsync). Awaited adapter calls stay outside the lock so
it is never held across I/O.

Client.Shared-006: HandleKeepAliveFailureAsync had only a non-atomic
state check guarding re-entry, so two bad keep-alives could each start a
failover loop, racing to dispose/replace _session and double-replaying
subscriptions. It now claims an atomic _failoverInProgress slot via
Interlocked.CompareExchange; a re-entrant call returns immediately. The
loop body moved to RunFailoverAsync, wrapped in try/finally that resets
the flag.

Tests: added KeepAliveFailure_ReentrantWhileFailoverInFlight_RunsFailoverOnce
and SubscribeAndUnsubscribe_ConcurrentCalls_DoNotCorruptState regression
tests; made the FakeSubscriptionAdapter / FakeSessionAdapter /
FakeSessionFactory test doubles thread-safe (and added a CreateGate hook)
so the concurrency tests exercise production locking rather than fake
state. All 138 Client.Shared tests pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/Client/ZB.MOM.WW.OtOpcUa.Client.Shared.Tests/Fakes/FakeSubscriptionAdapter.cs

@@ -12,6 +12,10 @@ internal sealed class FakeSubscriptionAdapter : ISubscriptionAdapter
         Dictionary<uint, (NodeId NodeId, Action<string, DataValue>? DataCallback, Action<EventFieldList>? EventCallback
             )> _items = new();
 
+    // Guards _items so concurrent-subscription tests exercise the production
+    // locking rather than tripping over the test double's own state.
+    private readonly object _itemsLock = new();
+
     private uint _nextHandle = 100;
     /// <summary>
     /// Gets a value indicating whether the fake subscription has been deleted.
@@ -34,7 +38,13 @@ internal sealed class FakeSubscriptionAdapter : ISubscriptionAdapter
     /// <summary>
     ///     Gets the handles of all active items.
     /// </summary>
-    public IReadOnlyCollection<uint> ActiveHandles => _items.Keys.ToList();
+    public IReadOnlyCollection<uint> ActiveHandles
+    {
+        get
+        {
+            lock (_itemsLock) return _items.Keys.ToList();
+        }
+    }
 
     /// <inheritdoc />
     public uint SubscriptionId { get; set; } = 42;
@@ -43,17 +53,24 @@ internal sealed class FakeSubscriptionAdapter : ISubscriptionAdapter
     public Task<uint> AddDataChangeMonitoredItemAsync(NodeId nodeId, int samplingIntervalMs,
         Action<string, DataValue> onDataChange, CancellationToken ct)
     {
-        AddDataChangeCount++;
-        var handle = _nextHandle++;
-        _items[handle] = (nodeId, onDataChange, null);
-        return Task.FromResult(handle);
+        lock (_itemsLock)
+        {
+            AddDataChangeCount++;
+            var handle = _nextHandle++;
+            _items[handle] = (nodeId, onDataChange, null);
+            return Task.FromResult(handle);
+        }
     }
 
     /// <inheritdoc />
     public Task RemoveMonitoredItemAsync(uint clientHandle, CancellationToken ct)
     {
-        RemoveCount++;
-        _items.Remove(clientHandle);
+        lock (_itemsLock)
+        {
+            RemoveCount++;
+            _items.Remove(clientHandle);
+        }
+
         return Task.CompletedTask;
     }
 
@@ -61,10 +78,13 @@ internal sealed class FakeSubscriptionAdapter : ISubscriptionAdapter
     public Task<uint> AddEventMonitoredItemAsync(NodeId nodeId, int samplingIntervalMs, EventFilter filter,
         Action<EventFieldList> onEvent, CancellationToken ct)
     {
-        AddEventCount++;
-        var handle = _nextHandle++;
-        _items[handle] = (nodeId, null, onEvent);
-        return Task.FromResult(handle);
+        lock (_itemsLock)
+        {
+            AddEventCount++;
+            var handle = _nextHandle++;
+            _items[handle] = (nodeId, null, onEvent);
+            return Task.FromResult(handle);
+        }
     }
 
     /// <inheritdoc />
@@ -80,7 +100,7 @@ internal sealed class FakeSubscriptionAdapter : ISubscriptionAdapter
     public Task DeleteAsync(CancellationToken ct)
     {
         Deleted = true;
-        _items.Clear();
+        lock (_itemsLock) _items.Clear();
         return Task.CompletedTask;
     }
 
@@ -89,7 +109,7 @@ internal sealed class FakeSubscriptionAdapter : ISubscriptionAdapter
     /// </summary>
     public void Dispose()
     {
-        _items.Clear();
+        lock (_itemsLock) _items.Clear();
     }
 
     /// <summary>
@@ -97,8 +117,13 @@ internal sealed class FakeSubscriptionAdapter : ISubscriptionAdapter
     /// </summary>
     public void SimulateDataChange(uint handle, DataValue value)
     {
-        if (_items.TryGetValue(handle, out var item) && item.DataCallback != null)
-            item.DataCallback(item.NodeId.ToString(), value);
+        (NodeId NodeId, Action<string, DataValue>? DataCallback, Action<EventFieldList>? EventCallback) item;
+        lock (_itemsLock)
+        {
+            if (!_items.TryGetValue(handle, out item)) return;
+        }
+
+        item.DataCallback?.Invoke(item.NodeId.ToString(), value);
     }
 
     /// <summary>
@@ -106,6 +131,12 @@ internal sealed class FakeSubscriptionAdapter : ISubscriptionAdapter
     /// </summary>
     public void SimulateEvent(uint handle, EventFieldList eventFields)
     {
-        if (_items.TryGetValue(handle, out var item) && item.EventCallback != null) item.EventCallback(eventFields);
+        (NodeId NodeId, Action<string, DataValue>? DataCallback, Action<EventFieldList>? EventCallback) item;
+        lock (_itemsLock)
+        {
+            if (!_items.TryGetValue(handle, out item)) return;
+        }
+
+        item.EventCallback?.Invoke(eventFields);
     }
 }