feat: add LangGraph checkpointer with OceanBase persistence support

[knqiufan]

Summary

This PR implements the LangGraph checkpointer for OceanBase, migrating from the deprecated ChatMessageHistory pattern to the modern state-based persistence approach.

close #46

Changes

1. New OceanBaseCheckpointSaver class (langchain_oceanbase/checkpointer.py)

• Implements BaseCheckpointSaver interface from LangGraph
• Core methods: put(), get(), get_tuple(), list(), put_writes()
• MySQL-compatible SQL adapted for OceanBase
• 4 database tables: checkpoint_migrations, checkpoints, checkpoint_blobs, checkpoint_writes
• Supports serialization/deserialization via JsonPlusSerializer

2. Deprecation warning for OceanBaseChatMessageHistory

• Added DeprecationWarning in __init__ method
• Points users to migration guide

3. Documentation (docs/migration_guide.md)

• Explains why to migrate (state-based vs message-based)
• Step-by-step migration instructions
• Code comparison (old vs new pattern)
• API reference and troubleshooting

4. Example (examples/langgraph_agent.py)

• Demonstrates persistent conversation memory
• Shows state recovery after restart
• Multi-thread session isolation

5. Integration tests (tests/integration_tests/test_checkpointer.py)

• Setup and table creation
• Put/Get operations
• List with filtering and limits
• Delete thread
• Version ID generation
• Full LangGraph integration tests

Dependencies

• Added langgraph-checkpoint >= 2.1.0, < 5.0.0 to pyproject.toml

[xxsc0529]

@knqiufan Thank you for your contribution. Can you resolve the PR conflict