fix: reject a provider named like the container fixture in expose()

[lesnik512]

Closes #67

expose() now raises ValueError when a provider's attribute name equals the container_fixture name in use (di_container by default, or whatever is passed). Before this, the generated fixture replaced the user's container fixture, every generated fixture then depended on itself, and each test failed with a recursive dependency error that did not point at the cause.

The error message names the group, the attribute, and the container fixture, and suggests renaming the provider or passing a different container_fixture.

Design notes:

• The check lives in _collect_fixtures, next to the existing duplicate-name check, so it runs before anything is installed on the module and can name the offending group. _collect_fixtures gains a keyword-only container_fixture argument defaulting to di_container, matching expose().
• I used plain ValueError for consistency with the duplicate-name collision. The modern-di exception types are about container registration and do not fit a fixture naming problem.
• A provider named di_container is still allowed when a different container_fixture is passed, since nothing collides in that case.
• Collisions with pytest built-in or other existing fixtures are out of scope (expose() silently overwrites a name already defined in the conftest #68).

Tests: three new cases in tests/test_expose.py (default name, custom name, and the no-collision case with a custom name). The first two failed before the fix. README's expose section mentions the new error.