Sync INDEXING_MAX_SEGMENTATION_TOKENS_LENGTH between API and Web

[fujita-h]

Summary

• Changed the INDEXING_MAX_SEGMENTATION_TOKENS_LENGTH setting to reflect the web side.
• Changed the default value of INDEXING_MAX_SEGMENTATION_TOKENS_LENGTH from 1000 to 4000.

Fixes: #11229

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