Fix script_utils invalid regex escape warning

[taivu1998]

Summary

Fixes #569 by making the parse_fasta header-splitting regex in openfold/utils/script_utils.py a raw string literal.

Root Cause

parse_fasta used the normal string literal '\W| \|'. On Python versions that warn about invalid escape sequences more aggressively, \W is interpreted as an invalid Python string escape before the value reaches the regex engine, producing:

SyntaxWarning: invalid escape sequence '\W'

The pattern is intended for re.split, so the regex escapes should be preserved directly.

Changes

• Converted the regex string in parse_fasta to a raw string literal.
• Added a lightweight compile-only regression test for script_utils.py that treats SyntaxWarning as an error without importing the heavier OpenFold runtime stack.

Validation

• python3.12 -W error::SyntaxWarning -m py_compile openfold/utils/script_utils.py
• python3.12 -m unittest tests.test_script_utils
• python3 -m unittest tests.test_script_utils
• git diff --check