fix: remove asyncio.run() from __del__ to prevent production crashes

[bleedblack1]

Fixes #154

Problem

The AgentGraph.__del__ method crashes in production:

def __del__(self) -> None:
    if self.pool:
        asyncio.run(self.close_pool())  #  RuntimeError!

Why it crashes: asyncio.run() cannot be called when an event loop is already running, which is always the case in Chainlit/uvicorn/FastAPI.

Secondary issue: LANGGRAPH_DB_URI is constructed at module import with potentially None values, creating malformed URIs.

Solution

Change	Purpose
Remove del	Eliminates the crash
Add aenter/aexit	Proper async context manager pattern
Add close() method	Explicit, idempotent cleanup
Add _build_langgraph_db_uri()	Lazy URI construction with validation
Add is_initialized/is_closed	State inspection properties
Add create_agent_graph()	Convenience factory function

Related Issues

• Fixes the asyncio.run() crash bug
• Addresses malformed URI construction
• Improves resource management patterns