Fix code review: use public variants property instead of private _variants in Catalog

Agent-Logs-Url: https://github.com/microsoft/Foundry-Local/sessions/8082508c-1338-48b2-bdd3-6c2c8e35e195

Co-authored-by: baijumeswani <12852605+baijumeswani@users.noreply.github.com>

Author: Copilot

sdk/python/src/catalog.py

@@ -171,7 +171,7 @@ def get_latest_version(self, model_or_variant: IModel) -> IModel:
 
         # variants are sorted by version, so the first one matching the name is the latest version.
         latest = None
-        for v in model._variants:
+        for v in model.variants:
             if v.info.name == model_or_variant.info.name:
                 latest = v
                 break