fix: cast idToken to str to satisfy mypy [no-any-return] on line 83

firebase is an untyped package (type: ignore[import-untyped]), so
self._hub_user['idToken'] resolves to Any. Since _get_auth_token()
is declared -> str, mypy raised:
  error: Returning Any from function declared to return 'str' [no-any-return]

Fix: wrap with str() cast which is always safe since Firebase idTokens
are JWT strings.

Author: amathxbt

src/opengradient/client/model_hub.py

@@ -80,7 +80,7 @@ def _get_auth_token(self) -> str:
             expires_in = int(refreshed.get("expiresIn", 3600))
             self._token_expiry = time.time() + expires_in
 
-        return self._hub_user["idToken"]
+        return str(self._hub_user["idToken"])  # cast Any->str for mypy [no-any-return]
 
     def create_model(self, model_name: str, model_desc: str, version: str = "1.00") -> ModelRepository:
         """