build(deps): bump langchain-community from 0.2.9 to 0.2.19 in /llm_demo

Bumps [langchain-community](https://github.com/langchain-ai/langchain) from 0.2.9 to 0.2.19.
- [Release notes](https://github.com/langchain-ai/langchain/releases)
- [Commits](langchain-ai/langchain@langchain-community==0.2.9...langchain-community==0.2.19)

---
updated-dependencies:
- dependency-name: langchain-community
  dependency-type: direct:production
...

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

llm_demo/requirements.txt

@@ -3,7 +3,7 @@ google-auth==2.33.0
 google-cloud-aiplatform[evaluation]==1.72.0
 itsdangerous==2.2.0
 jinja2==3.1.4
-langchain-community==0.2.9
+langchain-community==0.2.19
 langchain==0.2.11
 langchain-google-vertexai==1.0.7
 markdown==3.5.2