ptx: fix panic when truncation string/keyword contain multibyte Unicode

[Xylphy]

Fixes #10750

Summary
Fixes a ptx panic triggered by non-ASCII input (e.g., emoji) when calculating the “after” chunk width.

Root cause: max_after_size was computed using byte lengths (.len()), while the output chunking logic operates on characters. With multibyte Unicode, this can make max_after_size smaller than the number of characters actually appended, tripping assert!(max_after_size >= after.len()).

[github-actions]

GNU testsuite comparison:

Note: The gnu test tests/cut/bounded-memory is now being skipped but was previously passing.
Congrats! The gnu test tests/tail/tail-n0f is now passing!