fix(mqtt): a reconnect that re-establishes nothing tears the session down

Driver.Mqtt-007 (Low, error handling). OnReconnectedAsync throws when a
re-subscribe is granted zero filters — MqttConnection's contract is that a
throwing Reconnected subscriber tears the fresh session down and retries under
backoff, because a connected-but-deaf driver is the one outcome this path must
never serve. But when the live subscribed topics resolved to zero filters in the
first place it logged a warning and returned, reaching the same "nothing was
re-established" state by a different route and giving it the opposite answer.

Throw there too, and say so in the remarks so the type states ONE rule. It costs
no other tag — every live topic is in the set that resolved to nothing, so there
is nothing left flowing to protect, which is the whole reason the partial-
rejection case is exempt — and it is not permanent: the retry runs against
whatever Register has published by then.

Test reaches the state the only way production can: Register prunes
_subscribedTopics of the topics its new table no longer carries, but a SUBSCRIBE
already in flight records its own topic AFTER that prune ran, so a deploy landing
inside the round-trip leaves a live topic no authored tag resolves to. Paired
with the control that a reconnect with no live subscriptions at all stays a
no-op — throwing there would tear down the session of every driver whose tags
have not been subscribed yet.

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Mqtt.Tests/MqttSubscriptionManagerTests.cs

@@ -583,6 +583,51 @@ public sealed class MqttSubscriptionManagerTests
         mgr.Values.Read(OvenTempPath).StatusCode.ShouldBe(Good);
     }
 
+    /// <summary>
+    ///     <b>Driver.Mqtt-007.</b> Live subscribed topics that resolve to ZERO filters re-establish
+    ///     nothing, which is the same outcome as a total SUBACK failure and must get the same answer:
+    ///     throw, so <see cref="MqttConnection"/> tears the fresh session down and retries rather than
+    ///     serving a connected-but-deaf driver. A warning was not enough — the whole point of the type's
+    ///     split contract is that "no subscription was re-established" is fatal to the session.
+    /// </summary>
+    /// <remarks>
+    ///     The state is reached the only way production can reach it: <see cref="MqttSubscriptionManager.Register"/>
+    ///     prunes <c>_subscribedTopics</c> of the topics its new table no longer carries, but a SUBSCRIBE
+    ///     already in flight records its topic <i>after</i> that prune has run — so a redeploy landing
+    ///     inside the subscribe round-trip leaves a live topic no authored tag resolves to.
+    /// </remarks>
+    [Fact]
+    public async Task OnReconnectedAsync_LiveTopicResolvingNoFilter_Throws_RatherThanServingADeafSession()
+    {
+        var transport = new FakeTransport();
+        var mgr = NewManager(transport);
+        mgr.Register([Tag(OvenTempPath, $$"""{"topic":"{{OvenTopic}}","payloadFormat":"Scalar","dataType":"Float64"}""")]);
+
+        transport.DuringSubscribe = () => mgr.Register([]);   // the redeploy lands mid-round-trip
+        await mgr.SubscribeAsync([OvenTempPath], TimeSpan.FromSeconds(1), TestContext.Current.CancellationToken);
+        transport.DuringSubscribe = null;
+
+        await Should.ThrowAsync<InvalidOperationException>(
+            () => mgr.OnReconnectedAsync(TestContext.Current.CancellationToken));
+    }
+
+    /// <summary>
+    ///     The falsifying control: a reconnect with no live subscriptions at all re-establishes nothing
+    ///     because there is nothing to establish, and must stay a no-op. Throwing here would tear down
+    ///     the session of every driver whose tags have not been subscribed yet.
+    /// </summary>
+    [Fact]
+    public async Task OnReconnectedAsync_NoLiveSubscriptions_IsANoOp()
+    {
+        var transport = new FakeTransport();
+        var mgr = NewManager(transport);
+        mgr.Register([Tag(OvenTempPath, $$"""{"topic":"{{OvenTopic}}","payloadFormat":"Scalar","dataType":"Float64"}""")]);
+
+        await Should.NotThrowAsync(() => mgr.OnReconnectedAsync(TestContext.Current.CancellationToken));
+
+        transport.Calls.Count.ShouldBe(0);
+    }
+
     // ---------------------------------------------------------------------------------
     // C1 — wildcard-authored tags on the paths that reconstruct state from a FILTER string.
     // Delivery matches an incoming topic (concrete); reconnect + degrade key off the subscribed
@@ -898,10 +943,18 @@ public sealed class MqttSubscriptionManagerTests
         /// <summary>When set, the whole subscribe call throws instead of answering.</summary>
         public Exception? Throw { get; set; }
 
+        /// <summary>
+        ///     Runs inside the subscribe round-trip, so a test can land a concurrent
+        ///     <see cref="MqttSubscriptionManager.Register"/> in the window between its prune and this
+        ///     call recording its topic as live.
+        /// </summary>
+        public Action? DuringSubscribe { get; set; }
+
         public Task<IReadOnlyList<MqttSubscribeOutcome>> SubscribeAsync(
             IReadOnlyList<MqttTopicSubscription> filters, CancellationToken cancellationToken)
         {
             Calls.Add(filters);
+            DuringSubscribe?.Invoke();
             if (Throw is not null) throw Throw;
             IReadOnlyList<MqttSubscribeOutcome> outcomes =
                 [.. filters.Select(f => new MqttSubscribeOutcome(f.Topic, !Reject(f.Topic), Reject(f.Topic) ? "NotAuthorized" : "GrantedQoS1"))];