fix(sessions): replay-buffer gap edge cases, effective-config exposure, capacity-0 tests

#2: Replace afterSequence+1<oldestRetained with overflow-safe oldestRetained>0&&afterSequence<oldestRetained-1 to prevent ulong wrap at MaxValue falsely reporting gap=true.
#3: Add ReplayBufferCapacity and ReplayRetentionSeconds to EffectiveEventConfiguration and populate from EventOptions in GatewayConfigurationProvider.
#4: Add four new SessionEventDistributorTests covering capacity=0 gap/no-gap paths and the ulong.MaxValue boundary case.
#5: Update class-level <remarks> to describe the Task 3 replay ring buffer (capacity + age eviction, TryGetReplayFrom) rather than its absence.
#6: Add O(n)-is-acceptable comment at TryGetReplayFrom linear scan.
#8: Narrow no-replay 4-arg ctor to internal; InternalsVisibleTo already covers the test project.

src/ZB.MOM.WW.MxGateway.Server/Configuration/GatewayConfigurationProvider.cs

@@ -49,7 +49,9 @@ public sealed class GatewayConfigurationProvider(IOptions<GatewayOptions> option
                 AllowMultipleEventSubscribers: value.Sessions.AllowMultipleEventSubscribers),
             Events: new EffectiveEventConfiguration(
                 QueueCapacity: value.Events.QueueCapacity,
-                BackpressurePolicy: value.Events.BackpressurePolicy.ToString()),
+                BackpressurePolicy: value.Events.BackpressurePolicy.ToString(),
+                ReplayBufferCapacity: value.Events.ReplayBufferCapacity,
+                ReplayRetentionSeconds: value.Events.ReplayRetentionSeconds),
             Dashboard: new EffectiveDashboardConfiguration(
                 Enabled: value.Dashboard.Enabled,
                 AllowAnonymousLocalhost: value.Dashboard.AllowAnonymousLocalhost,