Improve typing on Duo Chat message handlers

The following discussion from !2502 (merged) should be addressed:

• @juhee.lee started a discussion: (+4 comments)

  question/nit: is it necessary to use unknown for these handler params? I noticed we are following this pattern in other handler functions but since DuoChatWebviewMessages has typed notification, wonder if it's still unknown.

Would the following snippet miss any case we are currently handling?