refactor: move model definitions into providers and extract resolve module

- Rename ImageProvider to BlackForestProvider, TextProvider to MistralProvider
- Add get_provided_models() abstract method to Provider base class
- Move model lists from models.py into each provider's get_provided_models()
- Add providers/registry.py to aggregate models from all providers
- Extract infer_required_capabilities and resolve_model from config.py to resolve.py
- Update tests to use new names and import paths

bulkgen/providers/__init__.py

@@ -12,6 +12,11 @@ from bulkgen.providers.models import ModelInfo
 class Provider(abc.ABC):
     """Abstract base for generation providers."""
 
+    @staticmethod
+    @abc.abstractmethod
+    def get_provided_models() -> list[ModelInfo]:
+        """Return the models this provider supports."""
+
     @abc.abstractmethod
     async def generate(
         self,