Stringify orjson mapping keys after unstructure hooks

[adenzhou1350]

The orjson converter promises to coerce mapping keys to strings, but a non-identity unstructure hook replaces that coercion. Passthrough hooks for Union[str, int] and Any can consequently leave integer keys, and dumps() raises TypeError: Dict key must be str.

Compose str() after the selected key hook. This retains custom hooks and base85 bytes conversion; the special string-enum branch is unchanged.

Add regressions for Union/PEP 604/Any/int keys, an attrs field, string enums, bytes, and custom hooks returning either integers or strings. Update HISTORY.

Validation:

• Unchanged baseline: 8 of the 13 new cases fail on Windows and Linux.
• Windows CPython 3.12: 25 relevant tests pass, including a fresh pre-publication rerun.
• Linux CPython 3.12: 80 relevant tests pass, 8 optional-backend tests skip; Ruff check and format pass.

The complete supported-Python matrix has not been run locally. This does not add lossless key round-tripping or collision handling.

AI-assisted implementation; the human submitter reviewed the complete patch and approved submission.