Add LangChain/LangGraph integration example

[sjsjsjjs534]

Summary

• Add a runnable LangChain/LangGraph integration example using OpikTracer and track_langgraph
• Include a dry-run path that exercises the same deterministic support-router logic without credentials
• Add focused tests for routing behavior and update the integration indexes

Test Plan

• UV_CACHE_DIR=/private/tmp/uv-cache-opik-examples-langchain uv run pytest -q
• UV_CACHE_DIR=/private/tmp/uv-cache-opik-examples-langchain uv run ruff check .
• UV_CACHE_DIR=/private/tmp/uv-cache-opik-examples-langchain uv run ruff format --check .
• UV_CACHE_DIR=/private/tmp/uv-cache-opik-examples-langchain bash run.sh
• UV_CACHE_DIR=/private/tmp/uv-cache-opik-examples-langchain uv run python -c "from langchain_langgraph import build_graph; app = build_graph(); print(app.invoke({'question': 'My login page throws an error'})['classification'])"

Closes #8