feat: add TTL removal support for pinning checkpoints (#66)

[bsbodden]

Add support for removing TTL from Redis checkpoints to make them persistent. This enables "pinning" specific threads that should never expire while allowing others to be
cleaned up automatically.

Changes:

• Add support for ttl_minutes=-1 parameter to trigger Redis PERSIST command
• Implement TTL removal in both sync and async checkpoint savers
• Apply PERSIST to main key and all related keys (blobs, writes)
• Add comprehensive test coverage for TTL removal functionality
• Update README with documentation for the pinning feature

[Copilot]

Pull Request Overview

This PR adds support for removing TTL from Redis checkpoints to enable "pinning" specific threads that should never expire while allowing others to be cleaned up automatically.

• Implements TTL removal using ttl_minutes=-1 parameter to trigger Redis PERSIST command
• Adds comprehensive test coverage for the TTL removal functionality
• Updates documentation with examples of the pinning feature

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
tests/test_ttl_removal.py	Comprehensive test suite covering TTL removal scenarios including sync/async operations and related keys
langgraph/checkpoint/redis/base.py	Implements TTL removal logic in sync checkpoint saver using Redis PERSIST command
langgraph/checkpoint/redis/aio.py	Implements TTL removal logic in async checkpoint saver and includes minor formatting fix
README.md	Documents the TTL removal feature with usage examples and updated feature description

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