core: add provider test coverage for upcoming refactor

Add comprehensive test suite for Provider module to ensure safe
refactoring of provider internals. Tests cover:
- Provider loading from env vars and config
- Provider filtering (disabled_providers, enabled_providers)
- Model whitelist/blacklist
- Model aliasing and custom providers
- getModel, getProvider, closest, defaultModel functions

Also adds Env module for instance-scoped environment variable access,
enabling isolated test environments without global state pollution.