refactor: cleanup provider base class; cleanup shared responsibilities; cleanup public contract

docs: document provider base class
refactor: cleanup custom provider, it should only deal with `is_custom` model configurations
fix: make sure openrouter provider does not load `is_custom` models
fix: listmodels tool cleanup

tests/test_dial_provider.py

@@ -121,7 +121,7 @@ class TestDIALProvider:
         """Test that get_capabilities raises for invalid models."""
         provider = DIALModelProvider("test-key")
 
-        with pytest.raises(ValueError, match="Unsupported DIAL model"):
+        with pytest.raises(ValueError, match="Unsupported model 'invalid-model' for provider dial"):
             provider.get_capabilities("invalid-model")
 
     @patch("utils.model_restrictions.get_restriction_service")