Align native datetime conversion with Python 3.15 - #9133

Summary

• Prefer __index__ for native date/datetime timestamp conversion; otherwise accept float-convertible values while preserving NaN/range checks and microsecond rounding.
• Validate ISO timezone hour/minute/second components before combining the offset.
• Apply final CPython's naive-local fold probe consistently, preserving main's Windows FILETIME support.

Split from #8954. Only crates/stdlib/src/_datetime.rs changes; no Lib, test, dependency, or lockfile changes. Compared against CPython final commit d220409a13b77de7a6c30abe16ea90db01528edc.

Validation

• Fresh matching narrow release build after cleaning exact workspace packages; threading,stdlib,stdio,importlib,host_env with default features disabled.
• Seven unchanged, source-hash-checked canonical datetime test methods passed in separate bounded processes: timestamp conversion (including Decimal/Fraction), UTC timestamp conversion, valid/invalid datetime and time ISO offsets, and low-timestamp fold detection. No skips or expected failures.
• Six direct protocol-order checks passed on both final CPython and the matching RustPython binary: __index__ precedence, index errors without float fallback, and float-only conversion, for date and aware datetime. All use small epoch values.
• Strict release Clippy passed for stdlib and VM, all targets, with matching explicit narrow features and -D warnings. Test binaries were compiled but not executed.
• Normal commit hooks passed, including rustfmt, cspell, and merge-conflict checks.

Validation above is bounded Linux coverage. Full datetime/workspace/C-API suites and Windows runtime behavior were not run locally. Hosted CI remains to be checked.

Implementation, source comparison, and validation were assisted by OpenAI Codex.

Assisted-by: OpenAI Codex:model version unavailable