[WIP] Implement tests for changes on main33

[Copilot]

• Understand the PR changes: new single-char separator fast path in MakeSeparatorListAny and new MakeSeparatorListVectorized(char c) with Vector512/256/128 SIMD paths
• Understand existing test structure in String.SplitTests.cs
• Add new test methods for string.Split(char, ...) with long strings at SIMD boundary lengths
  • Test lengths around Vector128 (Count=8, boundary=16), Vector256 (Count=16, boundary=32), Vector512 (Count=32, boundary=64)
  • Test separator positions: beginning, middle, end, multiple, consecutive, none, all
  • Test with low-value chars (packed path eligible) and high-value chars (non-packed path)
  • Test with various StringSplitOptions
• Build and run the tests to verify they pass
• Code review
• Security scan

Original prompt

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This section details on the original issue you should resolve

<issue_title>Implement tests for changes on main33</issue_title>
<issue_description></issue_description>

<agent_instructions>Implement tests for changes in dotnet#125379 (or equivalently between https://github.com/hamarb123/runtime/compare/78af1670568f72d0560ded17fc0310a95fe46590..b68c8681d0d0d3a047c96f937c1752cd9396d4a9 if you can't load the PR). Follow the suggestion of dotnet#125379 (comment), which is to add new tests to ensure the tests cover the new logic being added - I want tests for all of the changes. Either commit it directly to main33 in my fork if you're able, or make a PR in https://github.com/hamarb123/runtime for me to merge into main33 in my fork. Just add test cases to the relevant pre-exising tests that cover the relevant things to test.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Implement tests for changes on main33 #7

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