build(deps): bump langsmith from 0.5.19 to 0.5.20 (#466)

Bumps [langsmith](https://github.com/langchain-ai/langsmith-sdk) from
0.5.19 to 0.5.20.
<details>
<summary>Commits</summary>
<ul>
<li>See full diff in <a
href="https://github.com/langchain-ai/langsmith-sdk/commits">compare
view</a></li>
</ul>
</details>
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