[Bug]: Keyed scheduler runs sync tests serially on one thread, even with ParallelLimiter #7075

Bug report checklist

• I've searched existing issues to ensure this isn't a duplicate
• I've read the documentation and this isn't expected behavior
• I'm using the latest version of TUnit

Description

Synchronous tests with distinct NotInParallel keys run one at a time on a single thread.
Suspected cause: the scheduler starts each test inside its loop without yielding. Adding await Task.Yield() works around it.

Expected Behavior

~2s total, all test run in parallel

Actual Behavior

~8s total, T1 through T4 run serially, while Y1 through Y4 correctly run in parallel

Steps to Reproduce

using TUnit.Core.Interfaces;

[assembly: ParallelLimiter<Limit8>]

public class Limit8 : IParallelLimit { public int Limit => 8; }

public class WithoutYield
{
    [Test, NotInParallel("a1")] public void T1() => Thread.Sleep(2000);
    [Test, NotInParallel("a2")] public void T2() => Thread.Sleep(2000);
    [Test, NotInParallel("a3")] public void T3() => Thread.Sleep(2000);
    [Test, NotInParallel("a4")] public void T4() => Thread.Sleep(2000);
}

public class WithYield
{
    [Before(Test)] public async Task YieldHook() => await Task.Yield();

    [Test, NotInParallel("b1")] public void Y1() => Thread.Sleep(2000);
    [Test, NotInParallel("b2")] public void Y2() => Thread.Sleep(2000);
    [Test, NotInParallel("b3")] public void Y3() => Thread.Sleep(2000);
    [Test, NotInParallel("b4")] public void Y4() => Thread.Sleep(2000);
}

Inspect the test report to show which tests run in parallel.

TUnit Version

1.75.0

.NET Version

.NET 10.0

Operating System

Windows

IDE / Test Runner

dotnet CLI (dotnet test / dotnet run)

Error Output / Stack Trace

Additional Context

No response

IDE-Specific Issue?

• I've confirmed this issue occurs when running via dotnet test or dotnet run, not just in my IDE