gh-153296: Fix thread-safety data race in io.StringIO iterator

[Twix1288]

Summary

This PR fixes a TSAN-reported data race and segmentation fault when iterating over an io.StringIO object while it is being concurrently mutated (e.g., via write()).

Details

In the free-threaded build, io.StringIO memory safety is typically enforced automatically via Argument Clinic wrappers that acquire Py_BEGIN_CRITICAL_SECTION(self) for operations like readline and write.
However, the __next__ method triggered during loop iteration bypasses these wrappers. It is implemented in C as stringio_iternext, directly assigned to the tp_iternext slot. Previously, this slot accessed the string buffer (self->buf) and advanced self->pos without acquiring the object lock.
If another thread concurrently called write(), it would execute with the lock held but with its underlying self->pos mutating asynchronously. This led to out-of-bounds memset writes and use-after-free conditions.
This fix conceptually mirrors the implementation used for io.BytesIO: it splits the internal logic into stringio_iternext_lock_held and adds a wrapper stringio_iternext that properly secures the object's critical section.

• Issue: Use-after-free with a concurrent stringio_iternext #153296

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[Twix1288]

PR is done and tests are complete, should be good to go. Thanks!