fix: raise a clear RuntimeError when inject runs without setup_di

[lesnik512]

Problem

An @inject-decorated arq task that runs on a worker where setup_di(worker_settings, container) was never called dies inside the wrapper with:

KeyError: 'modern_di_request_container'

That is a private key name pointing at nothing; it says neither what is missing nor how to fix it.

Change

inject's wrapper now reads the per-job child through a private _fetch_child_container(ctx) helper that catches the KeyError and raises a plain RuntimeError with from None:

No modern-di container found for this job. Call setup_di(worker_settings, container) so jobs pass through the modern-di hooks before using @inject.

Only the lookup is wrapped; behaviour with setup_di in place is unchanged.

This follows the aiohttp/starlette/aiogram integrations (modern-python/modern-di-aiogram#22) and the rule in the integration guide, modern-python/modern-di#490: a package-local RuntimeError naming the fix, not a ModernDIError subclass.

Tests

• tests/test_jobs.py::test_inject_without_setup_di_names_the_fix: calls an @inject task with a bare ctx dict (no setup_di) and asserts RuntimeError matching setup_di(worker_settings, container). Written first; failed with the original KeyError before the fix.
• just lint and just test-ci pass (coverage 100%).