Bump langchain-google-genai from 2.1.5 to 2.1.8 (#5143)

Bumps [langchain-google-genai](https://github.com/langchain-ai/langchain-google) from 2.1.5 to 2.1.8.
- [Release notes](https://github.com/langchain-ai/langchain-google/releases)
- [Commits](langchain-ai/langchain-google@libs/genai/v2.1.5...libs/genai/v2.1.8)

---
updated-dependencies:
- dependency-name: langchain-google-genai
  dependency-version: 2.1.8
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

requirements.txt

@@ -5,7 +5,7 @@ imbalanced-learn==0.13.0
 langchain==0.3.26
 langchain-anthropic==0.3.15
 langchain-community==0.3.26
-langchain-google-genai==2.1.5
+langchain-google-genai==2.1.8
 langchain-mistralai==0.2.10
 langchain-openai==0.3.28
 libmozdata==0.2.10