refactor(sessions): remove the dead ISessionManager.ReadEventsAsync chain

ISessionManager.ReadEventsAsync had zero production call sites: the worker
event channel is drained once by GatewaySession.MapWorkerEventsAsync (the
distributor pump), and every consumer — gRPC subscribers, the dashboard
mirror, the alarm monitor — attaches to the distributor. The interface
member, SessionManager's forwarder, and GatewaySession.ReadEventsAsync are
gone; IWorkerClient/WorkerClient.ReadEventsAsync is untouched, it is the
live worker-channel claim.

No test was removed or rewired: nothing invoked the member through the
interface. Nine ISessionManager test fakes carried a required-member stub
(seven threw NotSupportedException or yielded nothing; EventStreamServiceTests
and GatewaySessionDashboardMirrorTests forwarded to the session; the two
MxAccessGatewayService fakes yielded their Events list) — all nine stubs were
deleted. The MxAccessGatewayService suites' streaming tests already run
through FakeEventStreamService, which reads the same Events list, so their
coverage is unchanged; only the now-inaccurate doc comments on Events /
LastReadEventsSessionId were reworded.

The MapWorkerEventsAsync comment no longer describes a twin to keep in step;
it now states the single-reader claim directly. docs/Sessions.md drops
ReadEventsAsync from the SessionManager member list and from the Run-state
prose. The 2026-08-15 deferred-remediation as-built note records the removal.

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Grpc/MxAccessGatewayServiceTests.cs

@@ -517,7 +517,7 @@ public sealed class MxAccessGatewayServiceTests
         /// <summary>The last owner key id passed to OpenSessionAsync.</summary>
         public string? LastOwnerKeyId { get; private set; }
 
-        /// <summary>The last session ID passed to ReadEventsAsync.</summary>
+        /// <summary>The last session ID the event stream service was asked to stream.</summary>
         public string? LastReadEventsSessionId { get; private set; }
 
         /// <summary>The last worker command passed to InvokeAsync.</summary>
@@ -540,10 +540,10 @@ public sealed class MxAccessGatewayServiceTests
         /// <summary>The number of times InvokeAsync was called.</summary>
         public int InvokeCount { get; private set; }
 
-        /// <summary>The events to return from ReadEventsAsync.</summary>
+        /// <summary>The events the fake event stream service replays for this manager.</summary>
         public List<WorkerEvent> Events { get; } = [];
 
-        /// <summary>Records the session ID passed to ReadEventsAsync.</summary>
+        /// <summary>Records the session ID the event stream service was asked to stream.</summary>
         /// <param name="sessionId">Identifier of the session.</param>
         public void RecordReadEventsSessionId(string sessionId)
         {
@@ -602,20 +602,6 @@ public sealed class MxAccessGatewayServiceTests
             return Task.FromResult(InvokeReply);
         }
 
-        /// <inheritdoc />
-        public async IAsyncEnumerable<WorkerEvent> ReadEventsAsync(
-            string sessionId,
-            [EnumeratorCancellation] CancellationToken cancellationToken)
-        {
-            LastReadEventsSessionId = sessionId;
-            foreach (WorkerEvent workerEvent in Events)
-            {
-                cancellationToken.ThrowIfCancellationRequested();
-                await Task.Yield();
-                yield return workerEvent;
-            }
-        }
-
         /// <inheritdoc />
         public Task<SessionCloseResult> CloseSessionAsync(
             string sessionId,