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Nice and clean implementation. I tested this with the tests (the new ones) of my implementation and they all passed (skipping the tests with chunkSize == 0).

The code could be sligthly more readable by having some vertical space to group related code, for instance after validating the input.

You could improve this a little bit by just having this

if (chunkSize == 1)
{
    yield return value;
    return;
}  

after the validation, in this way you wouldn't need to create a StringBuilder nor having the enumerator.