fix:position error when creating segments

[jarvis2f]

Summary

In the original code, the position of the segments was set using max_position + 1, which could lead to incorrect position calculation in scenarios with multiple concurrent requests or different datasets. Specifically, in a single request, the value of max_position might not have been correctly updated, resulting in an inaccurate calculation of the position for new segments.

Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods