fix: raise a clear RuntimeError when inject runs without the DI interceptor

[lesnik512]

Problem

An @inject-decorated servicer method (or a fetch_di_container() call) running on a server built without DIInterceptor / DIAioInterceptor failed with the raw ContextVar lookup error, which names nothing about the mistake or the fix:

LookupError: <ContextVar name='modern_di_request_container' at 0x...>

Over the wire the client saw StatusCode.UNKNOWN with details Exception calling application: <ContextVar name='modern_di_request_container' at 0x...> (sync) / Unexpected <class 'LookupError'>: <ContextVar ...> (aio).

Change

fetch_di_container and _resolve (the @inject path) now read the per-RPC child through one private helper, _current_container, which catches the LookupError and raises a plain RuntimeError from None:

No modern-di container found for this RPC. Add DIInterceptor (sync server) or DIAioInterceptor (aio server) to the server's interceptors so RPCs pass through it before using @Inject or fetch_di_container.

Behaviour with the interceptor in place is unchanged. The fetch_di_container docstring and the README API row are updated to say RuntimeError.

Behaviour change

fetch_di_container() outside an intercepted RPC now raises RuntimeError instead of LookupError. The one test that pinned LookupError (test_fetch_di_container_raises_outside_rpc) is changed to the new RuntimeError. The two white-box tests that assert LookupError directly on _request_container.get() are untouched, since the ContextVar itself is unchanged.

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

Tests

• tests/test_inject.py::test_inject_raises_without_interceptor (new): @inject method called with the ContextVar unset raises RuntimeError matching DIInterceptor.
• tests/test_inject.py::test_fetch_di_container_raises_outside_rpc: now expects RuntimeError matching DIInterceptor.
• tests/test_sync.py::test_inject_without_interceptor_reports_missing_interceptor (new): a real grpc.server with no interceptor; the client gets StatusCode.UNKNOWN with DIInterceptor in the details.

All three were written first and failed with the original LookupError before the fix. just lint-ci and just test-ci pass at 100% coverage.