test(sessions): document overflow race safety + close backpressure coverage gaps

- Issue 1: document the isOnlySubscriber snapshot race-safety assumption in
  OnSubscriberOverflow; flags the Task 7/8 revisit point explicitly.
- Issue 2: pin StreamDisconnects==1 in the FailFast overflow test so a
  regression dropping the StreamDisconnected("Detached") finally call is caught.
- Issue 3: replace plain int/bool? reads in SlowSubscriberOverflow test with
  Volatile.Read/Write + Interlocked.Increment stores to close the C# memory
  model data race on overflowCalls and observedIsOnlySubscriber.
- Issue 4: add SlowSubscriberOverflow_WithMultipleSubscribers_... distributor
  test pinning that isOnlySubscriber==false disables the session-fault path;
  includes TODO(Task 8) note for the GatewaySession-level assertion.
- Issue 5: reword SubscriberOverflowHandler XML doc to make explicit that the
  handler must NOT complete the subscriber's channel; the distributor owns that.

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Sessions/SessionEventDistributorTests.cs

@@ -336,7 +336,12 @@ public sealed class SessionEventDistributorTests
         // while a second, healthy subscriber keeps receiving and the pump keeps pumping.
         Channel<MxEvent> source = Channel.CreateUnbounded<MxEvent>();
         int overflowCalls = 0;
-        bool? observedIsOnlySubscriber = null;
+        // Separate fields for the bool value and the "set" flag so both can use
+        // Volatile.Read/Write; bool? is not valid for the volatile keyword on a local.
+        // Interlocked.Increment on the pump thread is the store for overflowCalls;
+        // Volatile.Read/Write provide ordering for observedIsOnlySubscriber.
+        int observedIsOnlySubscriberSet = 0;
+        bool observedIsOnlySubscriberValue = false;
         await using SessionEventDistributor distributor = new(
             "session-test",
             ct => source.Reader.ReadAllAsync(ct),
@@ -348,7 +353,8 @@ public sealed class SessionEventDistributorTests
             isOnlySubscriber =>
             {
                 Interlocked.Increment(ref overflowCalls);
-                observedIsOnlySubscriber = isOnlySubscriber;
+                Volatile.Write(ref observedIsOnlySubscriberValue, isOnlySubscriber);
+                Volatile.Write(ref observedIsOnlySubscriberSet, 1);
             });
         await distributor.StartAsync(CancellationToken.None);
 
@@ -372,8 +378,11 @@ public sealed class SessionEventDistributorTests
         Assert.Equal(SessionManagerErrorCode.EventQueueOverflow, fault.ErrorCode);
 
         // Two subscribers were registered at overflow time, so isOnlySubscriber is false.
-        Assert.Equal(1, overflowCalls);
-        Assert.False(observedIsOnlySubscriber);
+        // Use Interlocked.Read / Volatile.Read so the test-thread reads are ordered after the
+        // pump-thread writes, avoiding a data race by the C# memory model.
+        Assert.Equal(1, Volatile.Read(ref overflowCalls));
+        Assert.Equal(1, Volatile.Read(ref observedIsOnlySubscriberSet));
+        Assert.False(Volatile.Read(ref observedIsOnlySubscriberValue));
         Assert.Equal(1, distributor.SubscriberCount);
 
         // The pump is still running and the healthy subscriber still receives new events.
@@ -382,6 +391,73 @@ public sealed class SessionEventDistributorTests
         Assert.Equal(11ul, afterOverflow.WorkerSequence);
     }
 
+    [Fact]
+    public async Task SlowSubscriberOverflow_WithMultipleSubscribers_HandlerSeesIsOnlySubscriberFalse_OtherKeepsReceiving()
+    {
+        // Distributor-level pin for "FailFast with multiple subscribers degrades to
+        // disconnect-only (no session fault)": when the overflowing subscriber is NOT the
+        // sole subscriber, isOnlySubscriber is false, so a FailFast-wired handler must NOT
+        // fault the session. This test drives the distributor directly (without GatewaySession)
+        // with two subscribers and a FailFast-style overflow handler seam, overflows the slow
+        // one, and asserts (a) isOnlySubscriber==false, (b) the other subscriber keeps
+        // receiving, and (c) the pump keeps running — all without a GatewaySession.
+        //
+        // TODO(Task 8): add a GatewaySession-level "session stays Ready" assertion once
+        // multi-subscriber config is enabled by the Tasks 7/8 validator/guard change.
+        Channel<MxEvent> source = Channel.CreateUnbounded<MxEvent>();
+        bool handlerFiredWithFalse = false;
+        bool sessionFaultWouldBeCalled = false; // tracks if a FailFast path would fault
+        await using SessionEventDistributor distributor = new(
+            "session-multi-sub",
+            ct => source.Reader.ReadAllAsync(ct),
+            subscriberQueueCapacity: 2,
+            replayBufferCapacity: 0,
+            replayRetentionSeconds: 0,
+            NullLogger<SessionEventDistributor>.Instance,
+            TimeProvider.System,
+            isOnlySubscriber =>
+            {
+                if (!isOnlySubscriber)
+                {
+                    // Multi-subscriber: FailFast degrades to disconnect-only.
+                    Volatile.Write(ref handlerFiredWithFalse, true);
+                }
+                else
+                {
+                    // Single-subscriber: FailFast would fault the session — must not happen here.
+                    Volatile.Write(ref sessionFaultWouldBeCalled, true);
+                }
+            });
+        await distributor.StartAsync(CancellationToken.None);
+
+        // Slow subscriber: never reads, so capacity-2 channel overflows quickly.
+        using IEventSubscriberLease slow = distributor.Register();
+        // Healthy subscriber: drains every event promptly.
+        using IEventSubscriberLease healthy = distributor.Register();
+
+        // Drive enough events to overflow the slow subscriber's channel.
+        for (ulong sequence = 1; sequence <= 10; sequence++)
+        {
+            source.Writer.TryWrite(Event(sequence));
+            _ = await ReadOneAsync(healthy.Reader);
+        }
+
+        // Slow subscriber is disconnected with the overflow fault.
+        SessionManagerException fault = await Assert.ThrowsAsync<SessionManagerException>(
+            async () => await DrainUntilFaultAsync(slow.Reader));
+        Assert.Equal(SessionManagerErrorCode.EventQueueOverflow, fault.ErrorCode);
+
+        // The handler saw isOnlySubscriber==false (multi-subscriber degradation path).
+        Assert.True(Volatile.Read(ref handlerFiredWithFalse));
+        // The FailFast session-fault branch was NOT taken (session stays Ready equivalent).
+        Assert.False(Volatile.Read(ref sessionFaultWouldBeCalled));
+
+        // The pump and healthy subscriber are unaffected.
+        source.Writer.TryWrite(Event(11));
+        MxEvent afterOverflow = await ReadOneAsync(healthy.Reader);
+        Assert.Equal(11ul, afterOverflow.WorkerSequence);
+    }
+
     private static async Task DrainUntilFaultAsync(ChannelReader<MxEvent> reader)
     {
         // Drains any buffered events, then surfaces the channel's completion fault (if any)