Fix race condition in ReactiveCommand cancellation test (#4196)

Fixes an intermittent test failure in
`ReactiveCommandCreateFromTaskThenCancelSetsIsExecutingFalseOnlyAfterCancellationCompleteAsync`
caused by a race condition.

## Root Cause

The test was experiencing intermittent failures where `IsExecuting`
would be `False` when expected to be `True`. This occurred due to a race
condition where the subscription to the `IsExecuting` observable might
not receive the initial value before the command starts executing,
causing the first `IsExecuting = true` notification to be missed.

## Changes Made

Added synchronization using `TaskCompletionSource` to wait for the
`IsExecuting` observable to emit its first value before calling
`Execute()`. This ensures the subscription is fully established and has
received the initial state before execution begins, eliminating the race
condition reliably.

The fix waits for the first emission from `IsExecuting` (which should be
`false` initially) before starting command execution, guaranteeing that
subsequent emissions (including the `true` value when execution starts)
will be captured by the subscription.

## Testing

- ✅ Ran the failing test 30 times - all passed
- ✅ Ran all 72 ReactiveCommand tests - all passed (1 skipped)
- ✅ Build succeeds with .NET 8, 9, and 10 target frameworks

The fix is minimal and surgical, affecting only the test timing to
ensure reliable execution without changing any production code or test
assertions.

<!-- START COPILOT CODING AGENT SUFFIX -->



<details>

<summary>Original prompt</summary>

> ```
> Skipped ReactiveCommandCreateFromTaskHandlesExecuteCancellation [< 1
ms]
> Failed
ReactiveCommandCreateFromTaskThenCancelSetsIsExecutingFalseOnlyAfterCancellationCompleteAsync
[14 ms]
>   Error Message:
>      IsExecuting should be true when execution is underway
> Assert.That(Volatile.Read(ref latestIsExecutingValue), Is.True)
>   Expected: True
>   But was:  False
> 
>   Stack Trace:
> at
ReactiveUI.Tests.ReactiveCommandTest.ReactiveCommandCreateFromTaskThenCancelSetsIsExecutingFalseOnlyAfterCancellationCompleteAsync()
in
/home/runner/work/ReactiveUI/ReactiveUI/src/ReactiveUI.Tests/Commands/ReactiveCommandTest.cs:line
1826
> at
NUnit.Framework.Internal.TaskAwaitAdapter.GenericAdapter`1.BlockUntilCompleted()
> at
NUnit.Framework.Internal.MessagePumpStrategy.NoMessagePumpStrategy.WaitForCompletion(AwaitAdapter
awaiter)
> at
NUnit.Framework.Internal.AsyncToSyncAdapter.Await[TResult](TestExecutionContext
context, Func`1 invoke)
> at
NUnit.Framework.Internal.AsyncToSyncAdapter.Await(TestExecutionContext
context, Func`1 invoke)
> at
NUnit.Framework.Internal.Commands.TestMethodCommand.RunTestMethod(TestExecutionContext
context)
> at
NUnit.Framework.Internal.Commands.TestMethodCommand.Execute(TestExecutionContext
context)
> at
NUnit.Framework.Internal.Commands.BeforeAndAfterTestCommand.<>c__DisplayClass1_0.<Execute>b__0()
> at
NUnit.Framework.Internal.Commands.DelegatingTestCommand.RunTestMethodInThreadAbortSafeZone(TestExecutionContext
context, Action action)
> 
> 1) at
ReactiveUI.Tests.ReactiveCommandTest.ReactiveCommandCreateFromTaskThenCancelSetsIsExecutingFalseOnlyAfterCancellationCompleteAsync()
in
/home/runner/work/ReactiveUI/ReactiveUI/src/ReactiveUI.Tests/Commands/ReactiveCommandTest.cs:line
1826
> at
System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1.AsyncStateMachineBox`1.ExecutionContextCallback(Object
s)
> ```
> 
> Fix the build for the errors above, these are intermetient failures,
likely need a NoParallezize flag on the build, possibly a helper class
similar to those already in the Tests project to reset state
> 
> PREREQS needed - do these before anything else
> 1. Install .net8/9/10 before starting via the microsoft install
script.
> 2. Unshallow the commits
> 3. dotnet Workload restore in the /src folder.


</details>



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---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: glennawatson <[email protected]>

Author: Copilot

src/ReactiveUI.Tests/Commands/ReactiveCommandTest.cs

@@ -1811,14 +1811,27 @@ await Task.Delay(
         fixture.ThrownExceptions.Subscribe(_ => { });
 
         var latestIsExecutingValue = false;
+        var subscriptionReady = new TaskCompletionSource<bool>();
+        var isFirstValue = true;
+
         fixture.IsExecuting.Subscribe(isExec =>
         {
             statusTrail.Add((position++, $"command executing = {isExec}"));
             Volatile.Write(
                            ref latestIsExecutingValue,
                            isExec);
+
+            // Signal that we've received the first value (should be false initially)
+            if (isFirstValue)
+            {
+                isFirstValue = false;
+                subscriptionReady.TrySetResult(true);
+            }
         });
 
+        // Wait for the subscription to receive the initial value
+        await subscriptionReady.Task;
+
         var disposable = fixture.Execute().Subscribe();
 
         // Phase 1