update text splitter

[c121914yu]

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[sweep-ai]

Hey @c121914yu, here is an example of how you can ask me to improve this pull request:

@Sweep Add unit tests for the `commonSplit` function in `textSplitter.ts` to verify the new markdown splitting behavior, particularly testing:
- Splitting with different header levels (#, ##, ###, ####)
- Behavior with the new maxLen multipliers (1.2, 1.4, 1.6, 1.8)
- Cases where forbidConcat is true vs false
- Edge cases with very small chunks and the new minimum chunk size logic

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[c121914yu]

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