Fix: Convert scopes set to list in Credentials.__init__

This change ensures that if a set of scopes is passed to Credentials, it is converted to a list. This prevents issues with JSON serialization (to_json failure) and ensures consistent mutability behavior for the scopes property.

Also updated tests/test__oauth2client.py to handle the type change (set -> list) in equality checks.

Fixes #1145

Author: google-labs-jules[bot]

tests/test__oauth2client.py

@@ -56,7 +56,7 @@ def test__convert_oauth2_credentials():
     assert new_credentials._client_id == old_credentials.client_id
     assert new_credentials._client_secret == old_credentials.client_secret
     assert new_credentials._token_uri == old_credentials.token_uri
-    assert new_credentials.scopes == old_credentials.scopes
+    assert set(new_credentials.scopes) == set(old_credentials.scopes)
 
 
 def test__convert_service_account_credentials():