chore(deps): bump langchain-core from 0.3.71 to 0.3.81

Bumps [langchain-core](https://github.com/langchain-ai/langchain) from 0.3.71 to 0.3.81.
- [Release notes](https://github.com/langchain-ai/langchain/releases)
- [Commits](langchain-ai/langchain@langchain-core==0.3.71...langchain-core==0.3.81)

---
updated-dependencies:
- dependency-name: langchain-core
  dependency-version: 0.3.81
  dependency-type: direct:production
...

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

Author: dependabot[bot]

requirements.txt

@@ -1,4 +1,4 @@
 google-cloud-spanner==3.55.0
-langchain-core==0.3.71
+langchain-core==0.3.81
 langchain-community==0.3.27
 pydantic==2.11.7