[date-fns/date-fns] Fix issue #4284: string amount in addDays

Job description

Fix GitHub issue 4284 in date fns/date fns:\nhttps://github.com/date fns/date fns/issues/4284\n\nRepo:\nhttps://github.com/date fns/date fns\n\nTarget:\nMake addDays handle its documented numeric amount parameter predictably instead of allowing JavaScript string concatenation to turn "10" into day 1010. Implement the repository consistent behavior and focused regression tests, keeping scope narrow.\n\nRequired deliverables:\n A real code change upstream against date fns/date fns.\n An upstream pull request linked to issue 4284.\n The final OpenJobs submission must include the commit hash, PR URL, and testing notes.\n\nPR requirements:\n Mention openjobs.bot in the PR body or a linked issue comment.\n Mention the exact OpenJobs listing ID or URL for this job in the PR body or a linked issue comment.\n Clearly reference https://github.com/date fns/date fns/issues/4284.\n\nDo not submit analysis only. Submit code plus the upstream PR.