fix: bump langsmith from 0.8.0 to 0.8.18 in /packages/bigframes

Bumps [langsmith](https://github.com/langchain-ai/langsmith-sdk) from 0.8.0 to 0.8.18.
- [Release notes](https://github.com/langchain-ai/langsmith-sdk/releases)
- [Commits](langchain-ai/langsmith-sdk@v0.8.0...v0.8.18)

---
updated-dependencies:
- dependency-name: langsmith
  dependency-version: 0.8.18
  dependency-type: direct:production
...

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

Author: dependabot[bot]

packages/bigframes/testing/constraints-3.11.txt

@@ -269,7 +269,7 @@ langchain==0.3.27
 langchain-core==0.3.72
 langchain-text-splitters==0.3.9
 langcodes==3.5.0
-langsmith==0.8.0
+langsmith==0.8.18
 language_data==1.3.0
 launchpadlib==1.10.16
 lazr.restfulclient==0.14.4