fix(models) : string mutation bug in from_proteinnet_string

[aarushisingh04]

what does this PR do?

fixes a TypeError in openfold_utils/protein.py where from_proteinnet_string attempts to mutate an immutable string in-place.

the original code tried to assign seq[i] = "X" to handle unknown residues, which crashes. this pr replaces that logic with a list comprehension to safely sanitize the sequence, resolving the existing FIXME.

before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

who can review?

@ArthurZucker

[Rocketknight1]

Yup, makes sense! I doubt this code path was ever taken, but the fix is clean and obviously correct so we might as well include it.

[HuggingFaceDocBuilderDev]

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