SessionBindingTests: let readers confirm the live session before racing transfers (#4848)

## Problem

`BindingLivenessRacingTransfersAndCloseNeverConfirmsReplacedBindingAsync`
failed on the net48 CI runner (test-windows-net48 11/16, run
39250649914, on docs-only PR #4841) with `confirmed = 0`. The safety
check (`errors`) was empty. The reader tasks were not scheduled until
all 1000 transfers and the close had finished, so the liveness check
never saw a live binding.

## Fix (test only)
Before the first transfer, the test now waits until every reader has
confirmed the live binding at least once (bounded: 30 s, then a clear
`Assert.Fail`). The race still covers transfers and close, but the
positive assertion no longer depends on thread-pool scheduling. The
safety assertion is unchanged.

## Verification
I ran the test with a temporary `[Repeat(100)]` in Release builds:
- net10.0 and net48: 100 repeats each, no load
- net10.0 and net48: 3 × 100 repeats each with 2 busy loops per core

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

tests/Opc.Ua.Server.Tests/SessionBindingTests.cs

@@ -30,6 +30,7 @@
 using System;
 using System.Collections.Concurrent;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.Extensions.Time.Testing;
@@ -619,6 +620,7 @@ namespace Opc.Ua.Server.Tests
             int closed = 0;
             int checks = 0;
             int confirmed = 0;
+            int readersLive = 0;
             var errors = new ConcurrentQueue<string>();
             using var stop = new CancellationTokenSource();
 
@@ -629,6 +631,7 @@ namespace Opc.Ua.Server.Tests
                 readers[r] = Task.Run(() =>
                 {
                     int attempt = 0;
+                    bool seenLive = false;
                     while (!stop.IsCancellationRequested)
                     {
                         OperationContext channel = channels[(reader + attempt++) & 1];
@@ -650,6 +653,11 @@ namespace Opc.Ua.Server.Tests
                                 break;
                             }
                             Interlocked.Increment(ref confirmed);
+                            if (!seenLive)
+                            {
+                                seenLive = true;
+                                Interlocked.Increment(ref readersLive);
+                            }
                             if (closedBefore || binding.ActivationSequence < floor)
                             {
                                 errors.Enqueue(
@@ -662,6 +670,18 @@ namespace Opc.Ua.Server.Tests
 
             try
             {
+                // let every reader confirm the live session once before the race starts, so the
+                // positive check does not depend on the pool scheduling readers during transfers
+                var deadline = Stopwatch.StartNew();
+                while (Volatile.Read(ref readersLive) < kReaders && errors.IsEmpty)
+                {
+                    if (deadline.Elapsed > TimeSpan.FromSeconds(30))
+                    {
+                        Assert.Fail(
+                            $"Only {Volatile.Read(ref readersLive)} of {kReaders} readers confirmed the live session.");
+                    }
+                    await Task.Delay(1).ConfigureAwait(false);
+                }
                 for (int i = 1; i <= kTransfers; i++)
                 {
                     await ActivateAsync(created, channels[i & 1]).ConfigureAwait(false);