Deduplicate escape/quote scanning in f-string/t-string debug text handling

Parser/lexer/string.c has two copies of the same escape-and-quote scanning logic in _PyLexer_set_ftstring_expr (one loop to detect a comment, one to strip it). The two loops drifting apart is what caused gh-154711 / #154914. Follow-up: factor the shared scanning logic into a small helper so the two loops can't diverge again.

从 Parser/lexer/string.c 中的 _PyLexer_set_ftstring_expr 开始，比较它的两个转义和引号扫描循环：一个检测注释，另一个删除注释。将它们共用的扫描逻辑提取到一个小型辅助函数中，然后验证两条路径都使用它，不再逐渐偏离。