C API: Add PyUnicodeWriter_WriteRawStr() function

Feature or enhancement

Python 3.14 added PyUnicodeWriter_WriteStr() function. In general, writing str(obj) is fine. But there is one exception: writing a str subclass should not call str(obj), otherwise the output can be surprising if the subclass overrides __str__().

The _json and _io were impacted by the str subclass issue. They have been fixed by using the internal C API (_PyUnicodeWriter_WriteStr()). Problem: the internal C API is not usable outside CPython.

The PyUnicodeWriter_WriteStr() documentation has been modified to document a workaround for str subclasses:

To write a str subclass which overrides the __str__() method, PyUnicode_FromObject() can be used to get the original string.

Instead of the workaround, I propose adding a PyUnicodeWriter_WriteRawStr() function which handles str subclasses as expected (read the string content, don't use the __str__() method).

Linked PRs

• gh-150974

Review linked PR gh-150974 first, then read the existing PyUnicodeWriter_WriteStr documentation and the _json and _io references to _PyUnicodeWriter_WriteStr. Done means a public PyUnicodeWriter_WriteRawStr() API handles str subclasses without invoking str(), with the documented behavior available outside CPython internals.