4.0: tighten the provider surface #566

This was generated by AI during a pre-4.0 audit of main at 9cd88e7.

Observation

• The docs call the provider set closed, yet internal methods are public: AbstractProvider.mark_registered/get_dependencies/redirect_target/iter_validation_issues (providers/abstract.py:43-93), Factory.wiring_plan/can_call_positionally/resolution_step, Alias.find_source, CacheSettings.coerce. No integration calls them; once public in 4.0 they cannot change in a minor.
• Factory(cache=) accepts three spellings of "off": None, False, absent.
• AbstractProvider is an abc.ABC with no abstract methods (abstract.py:16). Besides misleading, it makes every isinstance a Python-level __instancecheck__ (98 ns vs 23 ns), hit per injected parameter in Container.resolve_dependency (container.py:156): Marker.resolve 260 -> 183 ns measured on 3.14.
• modern_di/__init__.py lists "providers" in __all__ without importing it; it works only because container.py imports modern_di.providers.abstract.

Proposal

• Underscore-prefix the internal methods (keep the public provider API: constructors, bound_type, scope, cast helpers, whatever the docs show).
• Narrow cache= to bool | CacheSettings.
• Drop abc.ABC from AbstractProvider.
• from modern_di import exceptions, integrations, providers.

Acceptance

Tests updated; Factory(cache=None) raises a clear error (or TypeError); migration guide entries; integrations still pass (modern-di-pytest imports AbstractProvider only as a type).