Ninjutsu must be able to activate only on/after declare blockers step

[JayDi85]

Bug report from #5859:

Ninjutsu wrong combat step - I can activate Ninjutsu before blockers are declared.

Oracle:

The ninjutsu ability can be activated only after blockers have been declared. Before then, attacking creatures are neither blocked nor unblocked.
(2022-02-18)
The ninjutsu ability can be activated during the declare blockers step, combat damage step, or end of combat step. In most cases (see below), if you wait until the combatdamage step or end of combat step, it will be after combat damage has been dealt, so the Ninja won't deal combat damage.
(2022-02-18)

If confirmed then potential fix: canPay method of the ability — check that current step index >= declare blocker step index.

[xenohedron]

Is this not covered correctly by UnblockedPredicate?