feat: Migrate to LangGraph Context API

[open-swe]

Fixes #8

Overview

This PR completes a comprehensive migration of the retrieval graph system to use the new LangGraph Context API. The changes replace the previous RunnableConfig-based configuration pattern with a more streamlined context-based approach.

Key Changes

• Updated Configuration and IndexConfiguration classes to support environment variable population and direct context usage
• Modified node functions in graph.py and index_graph.py to use runtime: Runtime[Context] parameters
• Updated retrieval functions to work with the new context API
• Refactored integration and unit tests to demonstrate the new usage pattern
• Updated README documentation to showcase the new context configuration approach

Migration Highlights

• Removed from_runnable_config() methods
• Added __post_init__() methods for environment variable handling
• Replaced config_schema with context_schema
• Updated function signatures to use runtime: Runtime[Context]
• Simplified configuration access through runtime.context

Breaking Changes

• Removed support for RunnableConfig-based configuration
• Changed how configuration is passed to graph methods
• Requires updates to existing code using the retrieval graph

Testing

• All existing test cases have been updated and verified
• Maintained full test coverage for user isolation and configuration handling
• Demonstrated compatibility with different retriever providers

This PR represents a significant improvement in configuration handling and aligns the project more closely with the latest LangGraph Context API patterns.