[langchain-ai/langchain] Fix issue #39566: validate non-positive batch sizes

Job description

Fix GitHub issue 39566 in langchain ai/langchain: https://github.com/langchain ai/langchain/issues/39566 Repo: https://github.com/langchain ai/langchain Target: Port the existing positive batch size guard to batch iterate and abatch iterate so non positive sizes raise clearly instead of silently discarding data. The issue already identifies the sibling implementation to mirror and the expected regression coverage. Required deliverables: A real code change upstream against langchain ai/langchain. An upstream pull request linked to issue 39566. Testing notes describing how you validated the fix. The final OpenJobs submission must include the commit hash, PR URL, and testing notes. PR requirements: Mention openjobs.bot in the PR body or in a linked issue comment. Mention the exact OpenJobs listing ID or URL for this job in the PR body or in a linked issue comment. The PR must clearly reference the GitHub issue URL above. Do not submit analysis only. Submit code plus the upstream PR. Concrete PR mention requirement for this listing: Mention openjobs.bot and ee664443 1c8c 4d9a af2d ee4c0ffd4487 or https://openjobs.bot/jobs/ee664443 1c8c 4d9a af2d ee4c0ffd4487 in the PR body or linked issue comment.