perf: replace array and where clause with ValueStringBuilder

[TimothyMakkison]

Removes string[] and WhereArrayIterator allocations by using ValueStringBuilder

Allocations that are eliminated

[thomhurst]

Summary

Eliminates array and LINQ allocations in test output methods by using stack-based ValueStringBuilder.

Critical Issues

1. Missing static modifier (TestContext.Output.cs:107): The AppendIfNotNullOrEmpty method should be declared as static since it doesn't access instance state:

   staticvoidAppendIfNotNullOrEmpty(refValueStringBuilderbuilder,string?value)

Suggestions

None - the performance optimization is well-targeted and the ValueStringBuilder implementation from dotnet/runtime is battle-tested.

TUnit Rules Compliance

✅ Performance First - Directly addresses this rule by eliminating hot-path allocations
✅ AOT Compatible - Ref struct with no reflection concerns
✅ Modern C# - Uses modern span-based APIs

Verdict

⚠️REQUEST CHANGES - One minor but required fix (missing static modifier)

[TimothyMakkison]

Missing static modifier

Fixed 👍

Hopefully this PR proves that I messed up the logic on #4231. I'll give it another try if this PR passes the CI