FixedString.Substring returns invalid UTF-8 when the string contains multi-byte characters

How to reproduce:

1. Create and open a new Universal 3D project
2. Ensure that the “com.unity.collections” package is installed
3. In the Project window, create a new “Editor” folder
4. Import the attached “FixedStringSubstringRepro” script into the created “Editor” folder (in the internal notes)
5. In the Menu bar, select “Tools” → “Bug Repros” → “FixedString Substring”
6. Observe the Console window

Actual result: The last log in the Console window is “rows=6 passed=2 flagged=4”
Expected result: The last log in the Console window is “rows=6 passed=6 flagged=0”

Reproducible with: 6000.0.0f1, 6000.7.0b6, 7000.0.0a10

Reproducible on: Windows 10 (by user), Windows 11
Not reproducible on: No other environments tested