fix: pass an optional request provider to choose_sa_engine for modern-di 4.0

[lesnik512]

Refs modern-python/modern-di#579.
Closes #56

Summary

modern-di 4.0 makes context values required. choose_sa_engine took request: litestar.Request[...] | None = None, filled from the integration's required litestar_request_provider, so resolving dynamic_engine outside an HTTP request (tests/test_db_routing.py::test_no_request_uses_primary) raises ContextValueNotSetError on 4.0.

Following the migration guide, Dependencies now declares its own optional_request (ContextProvider(litestar.Request, scope=Scope.REQUEST, bound_type=None, default=None)) and passes it to dynamic_engine explicitly.

default= on ContextProvider exists only in modern-di 4, so this PR needs modern-di 4. It lands together with bumping the modern-di-litestar pin to the release that allows modern-di 4. That release does not exist yet, so the pin is unchanged here.

CI on the current pin

CI fails until the pin bump, because modern-di-litestar>=3,<4 resolves modern-di 3.5.0, whose ContextProvider has no default parameter:

• lint: ty reports error[unknown-argument]: Argument `default` does not match any known parameter of `ContextProvider.__init__` at app/ioc.py:26.
• pytest: importing app.ioc raises TypeError: ContextProvider.__init__() got an unexpected keyword argument 'default'.

Verification

Local, Python 3.14, Postgres 14 in Docker, against the modern-di main wheel (the 4.0 candidate) installed over the synced env and run with uv run --no-sync: ruff format/check and ty clean, pytest 31 passed, coverage 100%. Before this change test_no_request_uses_primary failed with ContextValueNotSetError.

Stays draft until modern-di 4.0.0 and a modern-di-litestar release that allows it are on PyPI.

[lesnik512]

This was generated by AI.

Closing: modern-python/modern-di#602 lets a nullable or defaulted parameter fall back when its context value is unset, so the template's existing request: Request | None = None works on modern-di 4.0 without an extra provider. Checked: main at 0dd4326 against modern-di main 8cd5c07 passes ty and all 31 tests (100% coverage), including test_no_request_uses_primary.