Move hardcoded model fallback lists from module-level constants into
settings.fallback_models and settings.vision_fallback_models (pydantic
Settings fields). Can now be overridden via env vars
FALLBACK_MODELS / VISION_FALLBACK_MODELS or config/providers.yaml.
Removed:
- OLLAMA_MODEL_PRIMARY / OLLAMA_MODEL_FALLBACK from config.py
- DEFAULT_MODEL_FALLBACKS / VISION_MODEL_FALLBACKS from agent.py
get_effective_ollama_model() and _resolve_model_with_fallback() now
walk the configurable chains instead of hardcoded constants.
5 new tests guard the configurable behavior and prevent regression
to hardcoded constants.