Keep the 1.x handler registration forms as deprecated overloads

Consumers on 1.x wrote setRequestHandler(Schema, handler) and
setNotificationHandler(Schema, handler) with a whole-message Zod schema.
SDK 2.0 keys handlers by method name, so App and AppBridge now accept
that form as a third, deprecated overload: a small helper detects the
schema, extracts the method literal, forwards to the 2.x
(method, { params }, handler) form, and hands the legacy handler the
reassembled { method, params } message plus a 1.x-shaped extra
(signal, requestId, sessionId, _meta). A one-time console warning
points at the new form. The double-registration guard applies to both
forms.

The AppRequest, AppNotification and AppResult unions that 1.x fed to
the SDK's Protocol generics come back as deprecated type aliases, since
they leaked through the root entry and cost nothing to keep.

Both go away in 3.0.