gh-157407: Use _PyUnicode_AsUTF8NoNUL for AC 's' and 'z' converters (#158210)

* gh-157407: Use _PyUnicode_AsUTF8NoNUL for AC 's' and 'z' converters

The code was doing the same as the helper added in GH-111957 but inline.
That resulted in significantly more `.text` for most functions. Refactor
the Arguemnt Clinic converters code to deduplicate a bit while keeping
generated code close to original.

On my Linux x64 box this reduces `.text` by ~6200 bytes in a clang
release build (no PGO or LTO). Performance is flat.

The helper was already exported for `_sqlite3`, update the comment that
it is used in AC generated code as well.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* Update Include/internal/pycore_unicodeobject.h

Co-authored-by: Pieter Eendebak <pieter.eendebak@gmail.com>

* Remove NEWS, add period

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Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
Co-authored-by: Pieter Eendebak <pieter.eendebak@gmail.com>