Add tests for KeyString function

[turbolent]

Description

Ensure there are no accidental regressions in the KeyString functions.

Key strings are used when encoding dictionaries, so an accidental change without proper backwards-compatibility will lead to storage issues.

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For contributor use:

• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
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