.[agents] extra omits langchain-anthropic, so a shipped example cannot run #486

Files: pyproject.toml ([tool.poetry.extras]), affects examples/agents/langgraph/43_react_agent_multi_model.py

Symptom: After the documented install — pip install -e '.[agents]', which is what CI runs — the example fails immediately:

ModuleNotFoundError: No module named 'langchain_anthropic'

Cause: The agents extra lists langchain, langchain-core, langchain-openai, langgraph, google-adk, openai, openai-agents, anthropic, claude-code-sdk — but not langchain-anthropic, which 43_react_agent_multi_model.py imports for ChatAnthropic.

Fix: Add langchain-anthropic to the agents extra (and its own langchain-anthropic extra alongside the existing langchain / langgraph ones).

Verify: From a clean env, pip install -e '.[agents]' then python examples/agents/langgraph/43_react_agent_multi_model.py runs. Confirmed: with langchain-anthropic 1.5.5 installed manually it passes on both server types (5.8s / 76.7s).