fix: support async with on async persister factory methods

[andreahlert]

Summary

Fixes #546

AsyncSQLitePersister.from_values() and AsyncPostgreSQLPersister.from_values() are async classmethods that return coroutines. Using them directly with async with fails with:

TypeError: 'coroutine' object does not support the asynchronous context manager protocol

This PR introduces _AsyncPersisterContextManager, a thin wrapper that implements both __await__ and __aenter__/__aexit__, so factory methods now support both usage patterns:

# await (backwards compatible)
persister = await AsyncSQLitePersister.from_values(db_path="test.db")

# async with (what the issue requested)
async with AsyncSQLitePersister.from_values(db_path="test.db") as persister:
    await persister.initialize()
    ...

The same fix is applied to AsyncPostgreSQLPersister in b_asyncpg.py.

Test plan

• Reproduced the original TypeError in a Docker container (Python 3.11)
• Verified async with ... from_values() works after the fix
• Verified await ... from_values() still works (backwards compatibility)
• @skrawcz review
• @kajocina validate against original use case