Handle CSV writer callbacks safely - #9636

Summary

Use the existing callback-aware PyOperation for csv.writer.writerow instead of holding a non-reentrant unit mutex across iterator, field conversion, and write callbacks. A callback that invokes the same writer can otherwise wait on its own mutex. The writer's output buffer is local to each call, and its dialect and write target are immutable.

Obtain the row iterator through the normal iterator protocol and translate only iterator-acquisition TypeErrors to _csv.Error, as CPython does. Other acquisition errors and errors raised while advancing the iterator still propagate.

The row argument remains owned until after the operation section exits. This change introduces interpreter binding and operation bookkeeping; its runtime cost has not been benchmarked.

Scope

This addresses the lock/reentry behavior, not complete reentrant-output parity. CPython reuses an internal record buffer: a nested writerow from a field's __str__ can affect the outer record. RustPython constructs each record locally and already materializes input fields before string conversion, so its reentrant output and callback order can differ. Other iterator handling differences remain outside this change. Type names longer than 200 bytes are truncated at a UTF-8 boundary, which can differ from CPython at a multibyte cutoff.

This combines cleanly at file level with #9635; no merge was performed.

Validation

• Compared CPython 3.15.0 critical-section and writer implementation; a CPython callback reproduction confirmed a nested write from __iter__ returns and writes both records. A nested __str__ reproduction confirmed the buffer difference described above.
• CPython 3.15.0 reproductions covered TypeError and ValueError raised by __iter__, a non-iterator return, and TypeError from __next__.
• Independent source review of both commits and its composition with Require callable write method for CSV writers #9635, normal commit hooks, and diff checks passed. The RustPython deadlock follows from holding its non-reentrant mutex across callbacks; it was not reproduced in a local RustPython execution.
• RustPython was not built or executed locally. No tests were added or changed; hosted CI can run existing test_csv coverage.