Bump langchain-core in /examples/integrations/langgraph

Bumps [langchain-core](https://github.com/langchain-ai/langchain) from 0.3.80 to 0.3.81.
- [Release notes](https://github.com/langchain-ai/langchain/releases)
- [Commits](langchain-ai/langchain@langchain-core==0.3.80...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] <[email protected]>

Author: dependabot[bot]

examples/integrations/langgraph/requirements.txt

@@ -2,7 +2,7 @@
 # Pinned versions for reproducible builds across Mac/Windows/Linux
 
 # Core LangChain and LangGraph - compatible versions
-langchain-core==0.3.80
+langchain-core==0.3.81
 langchain-ollama==0.2.2
 langgraph==0.2.60