fix: keep the parent's child container when a sub-app also calls setup_di

[lesnik512]

Closes #45

When the parent app and a sub-app both call setup_di, both middlewares store the child container under the same request key. On a sub-app route the inner middleware deleted the key on exit, and the outer middleware's unconditional del then raised KeyError, so the response became a 500.

The middleware now reads the key's previous value before setting its own. On exit it puts that value back, or deletes the key when there was none. Deleting in the outermost middleware still breaks the request/container reference cycle, so the existing no-cyclic-garbage test is unchanged. A sub-app route resolves from the innermost app's container, and each middleware still closes the child container it opened.

Tests in tests/test_subapps.py:

• With setup_di on both apps, /x returns the parent's value and /sub/x returns the sub-app's, both 200.
• With setup_di on the parent only, /sub/x resolves from the parent's container. This one already passed before the fix and guards against a regression.
• A sub-app request runs the finalizers of both child containers. A test middleware on the parent resolves a cached request-scoped dependency from the parent's child container so it has something to finalize.

The first and third tests fail on main with the KeyError.

The README now says which container a sub-app route resolves from in each setup.