Tests were failing in CI because the models.json cache file doesn't exist
and the data() macro fallback only works at build time, not runtime.
The preload now pre-fetches models.json and disables the background
refresh to prevent race conditions during test execution.
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.
