Fix review comments for CSV string.Split benchmarks

[Copilot]

Addresses all review comments from the CSV string.Split benchmark PR. Merges the original changes and applies the requested fixes.

Changes

• Shorter CSV filenames: Renamed long-named files to avoid Windows path-length build failures. Original source URLs preserved in comments and THIRD-PARTY-NOTICES.TXT.

  • Monthly_Motor_Fuel_Sales_Reported_by_States__Selected_Data_from_FHWA_Monthly_Motor_Fuel_Report.csv → motor_fuel_sales.csv
  • 500_Cities__City-level_Data__GIS_Friendly_Format___2019_release.csv → 500_cities.csv

• Cached file I/O: CsvCorpus() previously called ReadInputFile() inline on every enumeration (BDN enumerates ArgumentsSource multiple times during discovery and per job). Replaced with static Lazy<string[]> fields loaded once on first access:

private static readonly Lazy<string[]> s_csvShortText = new Lazy<string[]>(() => ReadInputFile("TMEDTREND_PUBLIC_240528.csv"));
private static readonly Lazy<string[]> s_csvShortMixed = new Lazy<string[]>(() => ReadInputFile("motor_fuel_sales.csv"));
// ...

public static IEnumerable<object[]> CsvCorpus()
{
    yield return new object[] { "Short Text", s_csvShortText.Value };
    yield return new object[] { "Short Mixed", s_csvShortMixed.Value };
    // ...
}

• Style fix: new object []{ ... } → new object[] { ... } to match surrounding entries.

• Third-party notices: Added THIRD-PARTY-NOTICES.TXT documenting all six CSV sources (CMS, DOT, Census Bureau, USDA, EPA, CDC — all U.S. government public domain data).

[danmoseley]

@copilot push to the original pr we don't need this new one

[Copilot]

@copilot push to the original pr we don't need this new one

Unfortunately I'm only able to push to the branch of the PR I was invoked on (copilot/fix-comments-in-review-thread). I don't have the ability to push directly to pranas-string-split. You could cherry-pick the relevant commits from this branch onto the original PR, or the changes here could be referenced when merging.

[danmoseley]

Closing: review fixes have been cherry-picked onto PR #4659 (branch pranas-string-split).