Add the LangChain Replay integration

[Ekaterina Nikonova (ekaterina-nikonova)]

Overview

Adds langchain-replay, a testing library that records LangChain/LangGraph agent decisions and replays them while tools execute against the real filesystem.

Tests exercise real tool code paths without paying for LLM calls.

Type of change

Type: New documentation page

Checklist

• I have read the contributing guidelines, including the language policy
• I have tested my changes locally using docs dev
• All code examples have been tested and work correctly
• I have used root relative paths for internal links
• I have updated navigation in src/docs.json if needed

[github-actions]

Thanks for opening a docs PR, Ekaterina Nikonova (@ekaterina-nikonova)! When it's ready for review, please add the relevant reviewers:

• @mdrxy (Python integrations)

[Ekaterina Nikonova (ekaterina-nikonova)]

Mason Daugherty (@mdrxy) This is ready for review.