Preserve reflected addition for sequence subclasses - #9049

Summary

Extract the remaining sequence-subclass reflected-addition fix from #8954 (43065e79ddc46eafeca58cb39398ac007b5d25f7), adapted to the exact Python 3.14 slot contract on main 3af5f8a15314446d2b28a90e429c938dc7b8beeb.

Plain sequence subclasses inheriting a native concatenation descriptor must allow an unrelated right operand's __radd__. Preserve the distinction between inherited sequence concatenation and a numeric addition slot when methods are assigned, inherited, replaced, or deleted. Update the paired left slot when reflected addition changes, and use one stable numeric wrapper function identity.

The mega's unconditional treatment of native concatenation descriptors is insufficient for Python 3.14's stateful slot behavior. This patch preserves already-established numeric slots, exact descriptor identity/name, custom MRO membership, and unrelated numeric-subclass reflected priority. It follows CPython 3.14.7 type slot machinery.

Merged #8822 is already in the base and covers earlier override handling; this addresses the remaining inherited-native-sequence case rather than duplicating that change. No other direct duplicate appeared in the checked open/merged PR searches.

Validation

• Exact CPython 3.14.7: the complete updated existing builtin_str_add.py snippet passes.
• Unchanged native main fails the first new inherited-sequence regression; this head passes the entire snippet and its pytest entry.
• Both focused canonical descriptor binary-operator tests pass. Complete test_binop, test_augassign, and test_operator modules pass: 129 tests.
• Bounded differential probes match all 50 return/error-type outcomes and reflected call counts, plus the 35-row dynamic slot-state matrix. Some existing concatenation error strings differ from CPython; unchanged main produces those same diagnostic strings. This patch does not rewrite them.
• VM Clippy and separate C-API Clippy with warnings denied; normal repository commit hooks.
• Independent source review checked inheritance, exact native aliases, slot-wrapper identity, and the stored-MRO ownership check before native validation.

Coverage stays in the existing Python snippet; no Rust behavioral tests, canonical-test weakening, implementation-name guards, or README changes. Local checks are bounded Linux checks, not a complete workspace or cross-platform test claim. Full hosted CI is pending; this remains a draft for review.

AI-assisted implementation and validation with OpenAI Codex; model identifier unavailable. No human review is implied.

Summary by CodeRabbit

• Bug Fixes
  • Corrected addition behavior for subclasses of str, bytes, tuple, list, and bytearray, including when addition methods are inherited, overridden, assigned, or deleted.
  • Improved how regular and reflected addition interact with sequence concatenation, so operations follow Python’s method dispatch rules more consistently.