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/Grpc/EventStreamServiceTests.cs

@@ -206,6 +206,9 @@ public sealed class EventStreamServiceTests
         GatewayMetricsSnapshot snapshot = metrics.GetSnapshot();
         Assert.Equal(1, snapshot.QueueOverflows);
         Assert.Equal(1, snapshot.Faults);
+        // The finally block in StreamEventsAsync calls StreamDisconnected("Detached") on the
+        // overflow+fault path too; pin it here so a regression removing that call is caught.
+        Assert.Equal(1, snapshot.StreamDisconnects);
     }
 
     /// <summary>