numfmt: fix panic on multibyte locale decimal separator

[MadeNavaneeth]

Fixes #13937

What

numfmt panics with byte index N is not a char boundary when the input contains a multibyte locale decimal separator (e.g. Arabic ٫ U+066B in ar_SA.UTF-8) before a multibyte character and a suffix.

Root cause

find_valid_number_with_suffix used numeric_part.len() (byte length) as a char count in chars().skip(). When the numeric part contains multibyte characters, the byte count exceeds the char count, causing the char iterator to overshoot and the byte-slice index to land mid-character.

Fix

• Use numeric_part.chars().count() for the chars().skip() call
• Compute byte indices via char_indices().nth() for string slicing

Testing

• All 52 existing numfmt unit tests pass
• cargo clippy -p uu_numfmt -- -D warnings clean
• Manual test: LC_ALL=ar_SA.UTF-8 numfmt --from=si '1٫€K' now exits with error 2 instead of panicking

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/seq/seq-epipe is now passing!