fix: Event.message honors subclass field

_accept_convenience_kwargs already routes construction kwargs to a
subclass-declared `message` field, but the property/setter always used
`content`, so reads of that field returned the content alias instead of
the stored value. Defer to the field when present; base Event behavior
(message aliases content) is unchanged.

Change-Id: I8dd492407b8117ad9c6a90fb24269a03dac49ac0