v0.5.1 - LangGraph Subgraph Support & Performance Indexes

🐛 Critical Fix: LangGraph Namespaces

Previously, MemStateCheckpointer did not strictly isolate states based on checkpoint_ns (namespace).

• The Issue: If a parent graph and a subgraph shared the same thread_id, their states could overwrite each other or get mixed up upon restoration.
• The Fix: We now explicitly store and filter by checkpoint_ns. This guarantees full support for Nested Graphs and complex agent architectures.

⚡ Performance: New Indexes

We added specific database indexes to optimize state retrieval for LangGraph workflows.

• PostgreSQL: Added a B-Tree index on the JSONB path payload ->> 'checkpoint_ns'.
• SQLite: Added a functional index on json_extract(data, '$.payload.checkpoint_ns').

These indexes are created automatically when you initialize the storage.

🔄 Internals

• Updated put and aput methods in the Checkpointer to persist namespace metadata into the underlying Fact payload.