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/registry.py

@@ -0,0 +1,16 @@
+"""Aggregates models from all registered providers."""
+
+from __future__ import annotations
+
+from bulkgen.providers.models import ModelInfo
+
+
+def get_all_models() -> list[ModelInfo]:
+    """Return the merged list of models from all providers."""
+    from bulkgen.providers.blackforest import BlackForestProvider
+    from bulkgen.providers.mistral import MistralProvider
+
+    return (
+        MistralProvider.get_provided_models()
+        + BlackForestProvider.get_provided_models()
+    )