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>

Author: cmaloney

Include/internal/pycore_unicodeobject.h

@@ -450,7 +450,8 @@ extern void _PyUnicode_InternStatic(PyInterpreterState *interp, PyObject **);
 extern void _PyUnicode_ClearInterned(PyInterpreterState *interp);
 
 // Like PyUnicode_AsUTF8(), but check for embedded null characters.
-// Export for '_sqlite3' shared extension.
+// Export for '_sqlite3' shared extension, and for the Argument Clinic code
+// generated for the 's' and 'z' converters, which many shared extensions use.
 PyAPI_FUNC(const char *) _PyUnicode_AsUTF8NoNUL(PyObject *);
 
 

Misc/NEWS.d/next/Tools-Demos/2026-09-25-17-30-00.gh-issue-157407.iOVPov.rst

@@ -0,0 +1,3 @@
+Argument Clinic now calls the internal function ``_PyUnicode_AsUTF8NoNUL()`` for
+``s`` and ``z`` converters instead of writing equivalent code inline to reduce
+the generated ``.text`` segment.