Bump langsmith from 0.4.9 to 0.4.21

Bumps [langsmith](https://github.com/langchain-ai/langsmith-sdk) from 0.4.9 to 0.4.21.
- [Release notes](https://github.com/langchain-ai/langsmith-sdk/releases)
- [Commits](langchain-ai/langsmith-sdk@v0.4.9...v0.4.21)

---
updated-dependencies:
- dependency-name: langsmith
  dependency-version: 0.4.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

requirements.txt

@@ -2,7 +2,7 @@ beautifulsoup4==4.13.4
 langchain-couchbase==0.4.0
 langchain-community==0.3.27
 langchain-openai==0.3.28
-langsmith==0.4.9
+langsmith==0.4.21
 python-dotenv==1.1.1
 streamlit==1.47.1
 tqdm==4.67.1