feat!: require modern-di 4 and look up providers with Container.find_provider

[lesnik512]

Refs modern-python/modern-di#579.

Summary

• modern_di_grpc/main.py and tests/test_inject.py call container.find_provider(ServicerContext). modern-di 4.0 makes providers_registry private and adds find_provider as its public replacement.
• find_provider exists only in 4.0, so the requirement is now modern-di>=4,<5. This drops 3.x support, hence feat!.
• tests/dependencies.py: the comment on _make_context_reader described 3.x optional-context behaviour. In 4.0 a registered ContextProvider with no value raises ContextValueNotSetError, so the None default only applies in a container where no DIInterceptor registered grpc_context_provider. The comment now says that.

Verification

• 3.x: skipped. The lower bound is >=4 and find_provider does not exist in 3.x.
• 4.0: modern-di main (f300c2e) installed from a local wheel. ruff format/check, eof-fixer and ty pass; pytest 30 passed, coverage 100%.

CI will fail to resolve dependencies until modern-di 4.0.0 is on PyPI. This PR stays a draft until then.