* fix(providers): restore implicit model variant selection
Provider catalogs list only explicit overrides, so treating the first override as the default made base model behavior unreachable. Represent the base choice in the catalog and translate it back to an omitted variant at the provider boundary.
* test(providers): exercise variant catalog boundary
The regression concerns the model catalog callers receive, so verify normalization through fetchCatalog instead of coupling the test to its internal builder.
