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

[skrawcz]

Good solution to #546! The _AsyncPersisterContextManager approach is clean — supporting both await and async with from the same factory method is a nice UX improvement.

Two items to address:

1. Move _AsyncPersisterContextManager to burr/common/. Having b_asyncpg.py import from b_aiosqlite.py is an unwanted cross-dependency between unrelated integrations. Something like burr/common/types.py or a new burr/common/async_utils.py would be a better home.

2. Add tests for the async with pattern. All existing tests use await, so the new context manager functionality is untested. At minimum, a test like:

   async def test_async_sqlite_from_values_as_context_manager(tmp_path):
       db_path = str(tmp_path / "test.db")
       async with AsyncSQLitePersister.from_values(db_path=db_path) as persister:
           await persister.initialize()
           await persister.save("pk", "app1", 1, "pos", State({"k": "v"}), "completed")
           loaded = await persister.load("pk", "app1")
           assert loaded is not None

Also checked: AsyncRedisBasePersister.from_values is already a regular (non-async) method, so it doesn't have this issue. The fix correctly covers the only two affected persisters.

[andreahlert]

Good solution to #546! The _AsyncPersisterContextManager approach is clean — supporting both await and async with from the same factory method is a nice UX improvement.

Two items to address:

1. Move _AsyncPersisterContextManager to burr/common/. Having b_asyncpg.py import from b_aiosqlite.py is an unwanted cross-dependency between unrelated integrations. Something like burr/common/types.py or a new burr/common/async_utils.py would be a better home.
2. Add tests for the async with pattern. All existing tests use await, so the new context manager functionality is untested. At minimum, a test like:

   async def test_async_sqlite_from_values_as_context_manager(tmp_path):
       db_path = str(tmp_path / "test.db")
       async with AsyncSQLitePersister.from_values(db_path=db_path) as persister:
           await persister.initialize()
           await persister.save("pk", "app1", 1, "pos", State({"k": "v"}), "completed")
           loaded = await persister.load("pk", "app1")
           assert loaded is not None

Also checked: AsyncRedisBasePersister.from_values is already a regular (non-async) method, so it doesn't have this issue. The fix correctly covers the only two affected persisters.

Addressed all three points. While testing the fix in an isolated Docker container (Python 3.11), I caught two additional bugs:

• aexit would crash with AttributeError if aenter failed (e.g. bad db path), because self._persister was still None. Now guarded.
• The wrapper could be silently consumed twice (coroutine exhaustion). Added a _consumed flag that raises RuntimeError with a clear message.

Also fixed the parallelism.rst docs that were missing await on from_values() calls, and removed the now-redundant AsyncSQLiteContextManager helper from both test files.

LGTM. Could you please take an extra look?