feat: add MessageExporter for extracting conversation messages from c…

[bsbodden]

…heckpoints (#73)

• Implement MessageExporter class for extracting messages from Redis checkpoints
• Add LangChainRecipe for handling LangChain message formats (HumanMessage, AIMessage, etc.)
• Support both single checkpoint export and full thread export
• Use orjson for efficient JSON processing

[Copilot]

Pull Request Overview

This PR introduces a MessageExporter system for extracting conversation messages from Redis checkpoints, enabling easy export of message data for analysis or migration purposes.

• Adds MessageExporter class with support for single checkpoint and full thread exports
• Implements LangChainRecipe for handling various LangChain message formats
• Provides extensible recipe system for custom message formats

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
langgraph/checkpoint/redis/message_exporter.py	Core MessageExporter implementation with LangChainRecipe for message extraction
tests/test_message_exporter.py	Unit tests for MessageExporter and LangChainRecipe functionality
tests/test_message_exporter_integration.py	Integration tests demonstrating real-world usage scenarios
langgraph/checkpoint/redis/__init__.py	Exports new MessageExporter classes for public API

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