build(deps): update langchain-google-genai requirement

Updates the requirements on [langchain-google-genai](https://github.com/langchain-ai/langchain-google) to permit the latest version.
- [Release notes](https://github.com/langchain-ai/langchain-google/releases)
- [Commits](langchain-ai/langchain-google@libs/genai/v2.1.7...libs/genai/v2.1.8)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

pyproject.toml

@@ -32,7 +32,7 @@ langchain-openai = "0.3.27"
 langchain-anthropic = "0.3.17"
 langchain-community = "0.3.27"
 langchain-deepseek = "0.1.3"
-langchain-google-genai = "2.1.7"
+langchain-google-genai = "2.1.8"
 langchain-mcp-adapters = "0.1.9"
 langchain-huggingface = "0.3.0"
 langchain-ollama = "0.3.4"