Mirrors test_issue2025_xiaomi_env_key.py pattern:
- _PROVIDER_ENV_VAR['neuralwatt'] == 'NEURALWATT_API_KEY'
- _provider_has_key('neuralwatt') with env var set
- _provider_has_key('neuralwatt') without env var
- model group appears in get_available_models() when config lists models
