Restore Python 3.15 TextIOWrapper chunk-size deletion semantics - #9082

Summary

Restore final Python 3.15 TextIOWrapper._CHUNK_SIZE deletion behavior after the Python 3.14.8-specific change from #9071 landed alongside #8950.

• Check initialization and buffer attachment before rejecting deletion.
• Raise AttributeError("cannot delete attribute") for attached wrappers, including subclasses. Uninitialized and detached wrappers retain their precise ValueError contracts.
• Preserve every existing initialized, uninitialized, detached, ASCII-subclass, and non-ASCII-subclass regression case with exact final-reference exception types and messages. No skips or relaxed assertions are added.
• Leave assignment conversions, overflow/positive-size validation, and locking around the final write unchanged.

No files under Lib/ are changed. This is one shared-main correction; it does not duplicate the fix across the existing leaf PRs.

Reference: CPython v3.15.0 setter.

Failure evidence

The unrelated workflow-only #8958 fails the CPython 3.15 stdlib_io.py reference test on Windows and macOS. #9058 shows the same failure on Linux. The assertion was introduced by #9071, not the leaf changes. Other tests in those completed snippet jobs pass.

Validation

Candidate: ef1197a5e2477b719670768089033af1098a41ee, directly on main 06007c81fa99e813cbf4583dab326764ba0ef017.

• Normal commit hooks, whitespace checks, and independent source/coverage review pass.
• A certified, unpatched CPython build from the exact v3.15.0 source passes the bounded existing chunk-size cases. A separate 12-case in-memory probe confirms attached/closed/uninitialized/detached behavior for the base class and both subclass names.
• A fresh native interpreter build at ef1197a passes the same bounded existing cases and matches all 12 reference outcomes exactly. All workspace-package artifacts were cleared before rebuilding; the binary reports the expected commit and source/binary fingerprints were recorded.
• Warnings-denied VM-library and root-interpreter Clippy pass. No broad local suite was executed, and hosted cross-platform CI is pending.

AI assistance

OpenAI Codex assisted with diagnosis, implementation, independent review, and validation. The runtime does not expose a precise model identifier; the commit includes an Assisted-by trailer.