Fix string addition and sequence subclass dispatch - #8822

• This PR follows our AI policy

Summary

A str subclass that declines addition still gets concatenated:

class S(str):
    def __add__(self, other):
        return NotImplemented

S("a") + "b"

CPython raises TypeError; RustPython returns "ab". Direct calls also differ: str.__add__("a", 1) returns NotImplemented instead of raising TypeError.

Use the sequence concatenation slot for strings and leave reflected calls to the operator dispatcher. This removes the extra str.__radd__ method and prevents a declining right operand from being called twice. Clear inherited sequence concatenation when a subclass overrides __add__, and restore it when the native method becomes visible again.

The regression covers direct calls, operator.add, operator.iadd, reflected-call counts, and inherited or dynamically replaced overrides for strings, bytes, tuples, and lists. The existing exact-string bytecode fast paths remain unchanged.

Related: #7293 introduced the numeric string-addition slot; #8652 fixed concatenation errors when a reflected call declines. This preserves those error messages while correcting dispatch.

Validation

• The new regression passes on CPython 3.14.7 and 3.16, fails on unmodified RustPython main (393689d), and passes with this patch.
• test_str, test_operator, test_descr, test_exceptions, test_list, test_tuple, and test_bytes: 941 tests run, 59 skipped; all modules pass.
• extra_tests: 464 passed. Four failures also reproduce without the patch: the default build lacks _sqlite3, and three host-side multiprocessing tests return non-None values, which pytest rejects.
• C-API tests: 112 passed with debug assertions enabled.
• Formatting and configured commit hooks pass.
• Workspace Rust tests and doctests: 1,310 passed, 16 ignored (cargo test --release --workspace --exclude rustpython_wasm --exclude rustpython-venvlauncher --exclude rustpython-capi). The local macOS run needs the installed library directories in DYLD_FALLBACK_LIBRARY_PATH.
• Workspace and C-API Clippy pass.
• The contributor manually ran the regression successfully.

AI assistance

Codex assisted with investigation, implementation, regression tests, validation, and this description.

Summary by CodeRabbit

• Bug Fixes
  • Corrected string addition so non-string operands raise the appropriate concatenation error without calling their reflected addition method.
  • Fixed how concatenation behavior is resolved for sequence types, including when subclasses override or remove addition methods.
  • Improved consistency for direct and in-place addition, including cases where subclasses return NotImplemented.