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.
- fix scrollbox empty/blank last items at bottom
- fix should not insert chars with modifiers in input/textarea anymore
- do not wrap OSC4 palette sequences for tmux 3.6