fix: raise a clear RuntimeError when inject runs without setup_di

[lesnik512]

Closes #13.

Problem

An @inject-decorated dialog getter or callback reached by an update on a dispatcher that never had setup_di called failed with KeyError: 'modern_di_container' — a private key name pointing at nothing. The handler path in main.py had the identical failure (aiogram's _prepare_kwargs silently drops parameters missing from data, so the wrapper's kwargs.pop raised the same KeyError), so this PR fixes both.

Change

One private helper, _fetch_child_container, next to _CHILD_CONTAINER_KEY in main.py. It reads the per-update child from a mapping and converts KeyError into a RuntimeError that names the cause and the fix:

No modern-di container found for this update. Call setup_di(dispatcher, container) so updates pass through the modern-di middleware before using @Inject.

The handler inject wrapper and both branches of dialog._container_from_call go through it. Behaviour with setup_di in place is unchanged.

Why package-local, not a modern_di exception

The issue left this open. modern-di-aiohttp (fetch_request_container) and modern-di-starlette (inject) already raise a plain RuntimeError with this same wording pattern, and modern_di.integrations is deliberately exception-free — ModernDIError covers registration, resolution and lifecycle, not framework wiring. A shared error would need a core release and a floor bump here for no gain in this fix. Unifying across integrations is a reasonable later cross-repo change, out of scope here.

Tests

Three new tests, written failing first (each reproduced the original KeyError): handler path via a bare Dispatcher, dialog getter, and dialog callback (on_start, the 2-arg branch). Coverage stays at 100%.