perf: remove unused value and early return in DedThreadExe

[TimothyMakkison]

• Remove unsued DedicatedThreadTaskScheduler _taskScheduler field
• Make ExecuteAsync sync
• Make Task.Factory.StartNew sync

Before

After

[thomhurst]

Looks like ya posted the same image for before and after

[TimothyMakkison]

Looks like ya posted the same image for before and after

Yeah, I've fixed it, but semi abadoned this PR when I realised the savings weren't adding up and I had no idea how to prove that line 82 did anything.

[TimothyMakkison]

This should help a little, don't know how much though

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

---

Review Summary:

This PR makes clean performance optimizations that align well with TUnit's "Performance First" principle:

1. Removes unused field - The _taskScheduler field was stored but never used
2. Eliminates async overhead - Converting ExecuteAsync from async to sync by returning ValueTask directly avoids unnecessary state machine allocations
3. Optimizes Task.Factory.StartNew - Replacing the async lambda with a sync lambda + .AsTask() reduces allocations while maintaining identical semantics

All changes are semantically equivalent to the original code while reducing memory allocations in hot-path test execution code.