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

tests/test_builder.py

@@ -27,6 +27,11 @@ WriteConfig = Callable[[dict[str, object]], ProjectConfig]
 class FakeProvider(Provider):
     """A provider that writes a marker file instead of calling an API."""
 
+    @staticmethod
+    @override
+    def get_provided_models() -> list[ModelInfo]:
+        return []
+
     @override
     async def generate(
         self,
@@ -43,6 +48,11 @@ class FakeProvider(Provider):
 class FailingProvider(Provider):
     """A provider that always raises."""
 
+    @staticmethod
+    @override
+    def get_provided_models() -> list[ModelInfo]:
+        return []
+
     @override
     async def generate(
         self,