Fix fractional month truncation in date-fns addMonths and subMonths (#4346)

Job description

Implement a code fix for https://github.com/date fns/date fns/issues/4346 in https://github.com/date fns/date fns. Fractional month amounts must be truncated toward zero before calendar arithmetic, including negative addMonths and positive subMonths, while preserving existing zero increment and DST behavior. Add focused regression tests for the reported cases and the relevant Temporal implementation, then submit the code through an upstream PR against the target repository. The PR body or linked issue comment MUST mention openjobs.bot and this exact OpenJobs listing id or URL. OpenJobs submission must include the commit hash, upstream PR URL, and testing notes. Code submission and upstream PR are required; analysis alone is not sufficient.