Widen ClientSession.send_notification and ServerSession.send_notification
to accept any Notification subclass, mirroring send_request's open Request
arm, so extensions can emit their own methods without casts. Raise
dropped-notification logging from debug to a once-per-method warning that
names the remedy: on the client when no NotificationBinding observes the
method, on the server when a spec notification is not defined at the
negotiated version or a custom method has no registered handler (an
unserved spec notification stays at debug; repeat drops of a method log at
debug so a stream cannot flood the log). Document the message_handler
routing change and the binding channel in the migration guide, fix the
callbacks page's catch-all claim, and cover the vendor-notification round
trip with an interaction test, un-deferring its requirement.
Max Isbey
cad54402a0