feat: add manage_lifespan to setup_di

[lesnik512]

Closes #77

setup_di(app, container) gets a keyword-only manage_lifespan: bool = True, matching modern-di-fastmcp. With the default nothing changes. With manage_lifespan=False the app never opens or closes the root container, so the user can own it, for example with async with container: around running the app. Providers are still registered and the middleware still builds and closes a request container per message.

The container.open hook now registers after install_middleware instead of before it. Both run on startup before the broker consumes, so the order only shows if install_middleware raises for a missing broker.

README: the flag is in the API table, plus a short "Sharing a container" section. CONTEXT.md: the root container entry now says its app-tracked lifetime is the default.

Tests in tests/test_lifespan.py, written first and failing before the change:

• with the flag off, a full startup, one message and shutdown leave the root open and its APP finalizer unrun
• with the flag off, a request-scoped dependency still resolves and its request container is closed (its finalizer runs once)
• with the flag off and the root in async with, the APP finalizer runs once, at the user's exit

The existing restart test covers the default.