feat: add langchain-anthropic provider package to langchain example

[devin-ai-integration]

Summary

Adds langchain-anthropic to the LangChain example's dependencies so the example ships with Anthropic support out of the box, alongside the existing OpenAI, Google GenAI, and AWS Bedrock providers.

Also updates the README to list all included providers and explain how to add more (e.g. poetry add langchain-mistralai).

Review & Testing Checklist for Human

• Verify poetry install succeeds in getting_started/langchain/invoke/
• Verify the langchain example runs with an Anthropic model configured in the AI config

Link to Devin session: https://app.devin.ai/sessions/463c0aff13bf4335ba0c0ab765742847
Requested by: @ld-paul

[devin-ai-integration]

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