Bump langchain-core from 0.1.17 to 0.1.18 (#70)

Bumps [langchain-core](https://github.com/langchain-ai/langchain) from
0.1.17 to 0.1.18.
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Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>