Remove dynamic installation of built-in plugins. GitLab auth is now imported
directly as an internal plugin, eliminating network requests during startup
and simplifying the plugin loading logic.
Removes the need for test mocks since plugins are no longer
dynamically installed at runtime.
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.
