feat: add manage_lifespan flag to setup_di

[lesnik512]

Closes #39

setup_di(broker, container) gains a keyword-only manage_lifespan: bool = True, matching modern-di-fastmcp's flag of the same name.

With the default, nothing changes: WORKER_STARTUP opens the root container and WORKER_SHUTDOWN closes it. With manage_lifespan=False, setup_di skips registering those two handlers. It still attaches the container to broker.state and registers taskiq_message_provider, and each task still builds and closes its own child container. The caller owns the root, for example with async with container: around the apps that share it.

Skipping the handler registration leaves the default path unchanged, and the flag adds no work per worker cycle.

Docs:

• README: the flag in the API table and a short "Sharing a container" section.
• CONTEXT.md: the "Root container" entry no longer says the worker events always open and close it.

Tests in tests/test_lifespan.py, all with manage_lifespan=False:

• a full worker cycle with one task leaves the root open and its APP finalizer unrun
• an APP- and a REQUEST-scoped dependency resolve, and the REQUEST finalizer runs when the task ends
• with the caller owning the root in async with, the APP finalizer runs once, at the caller's exit

With the flag accepted but ignored, the three tests fail on their assertions. The existing lifespan tests cover the default and are unchanged.