Oath of Kaya must be reworked to support multiple triggers on multiple planeswalkers

Looks like [[Oath of Kaya]] wrongly implemented. It used `DECLARED_ATTACKERS` but it has additional oracle text about multiple triggers:

> If an opponent attacks more than one planeswalker you control, the last ability of Oath of Kaya triggers once for each of those attacked planeswalkers. (2019-05-03)


So it must be checked and reworked

Locate the Oath of Kaya implementation and its use of DECLARED_ATTACKERS. Compare the current trigger handling with the quoted Oracle text, focusing on attacks against multiple planeswalkers. Done means the card supports the specified multiple triggers and its behavior is covered by the relevant tests, though no test file is named in the issue.

Written by the indexing model from the issue text.