perf: replace SubsString with alternative lookup

[TimothyMakkison]

Use GetAlternateLookup to avoid allocaing a string via SubString.

Edit: use preprocessor directive, apparently you can't polyfill GetAlternativeLookup

Small improvement saves around 200KB

[thomhurst]

Summary

Replaces Substring allocation with GetAlternateLookup on .NET 9+ in test discovery hot path, saving approximately 200KB.

Critical Issues

None found - approved

Suggestions

None - this is a well-executed performance optimization that targets a hot path per Rule 4, uses preprocessor directive for backward compatibility, maintains semantic equivalence with zero-allocation lookup, and doesn't require snapshot updates.

The use of GetAlternateLookup with ReadOnlySpan is the correct modern approach for span-based HashSet lookups in .NET 9+.

Verdict

APPROVE - Clean performance win with no downsides