Update dependency langchain to v0.2.8

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
langchain (source)	0.2.7 -> 0.2.8

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Release Notes

langchain-ai/langchainjs (langchain)

### [`v0.2.8`](https://togithub.com/langchain-ai/langchainjs/compare/0.2.7...91ec5ba129ece755d60f6ed33d35d7ef92e29bd8) [Compare Source](https://togithub.com/langchain-ai/langchainjs/compare/0.2.7...056b871d3e27c66cc3cb7290d92579219234ec9f)

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Configuration

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