feat: support modern-di 4

[lesnik512]

Refs modern-python/modern-di#579.

Summary

• Widen the requirement to modern-di>=3.1,<5.
• tests/dependencies.py: fetch_request_path took request: flask.Request | None = None, with a comment saying the default made container.validate() treat the request as optional. In 4.0 a registered ContextProvider with no value raises regardless of the creator default, and the only test that resolves the helper does so inside a request. The parameter is now required and the comment is gone.
• tests/conftest.py: the fixture comment named the open lifecycle as a 3.x rule. 4.0 keeps it, so the comment no longer names a version.

Verification

• 3.x (modern-di 3.6.0 from PyPI): eof-fixer, ruff format/check and ty pass; pytest 15 passed, coverage 100%.
• 4.0 (modern-di main f300c2e from a local wheel): ty passes; pytest 15 passed, coverage 100%.

This PR stays a draft until modern-di 4.0.0 is on PyPI.