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.

docs/migrate-to-2.md

@@ -41,8 +41,9 @@ its `Protocol` class); `@modelcontextprotocol/core` is a required peer that `cli
   Valibot. Raw zod shapes (`{ q: z.string() }`) still work with
   `registerAppTool` but are deprecated; wrap them with `z.object({...})`.
 - **`App` / `AppBridge` extend `Protocol` from `@modelcontextprotocol/client`.**
-  `ProtocolWithEvents`, `AppRequest`, `AppNotification` and `AppResult` are
-  gone; use the SDK's `Protocol`, `Request`, `Notification` and `Result`.
+  `ProtocolWithEvents` is gone; use the SDK's `Protocol`. The `AppRequest`,
+  `AppNotification` and `AppResult` unions are kept as deprecated type
+  aliases (nothing in 2.x consumes them) and will be removed in 3.0.
 - **Handler context.** Custom handlers receive the SDK 2.x context:
   `extra.signal` → `extra.mcpReq.signal`, `extra.requestId` →
   `extra.mcpReq.id`.
@@ -51,7 +52,11 @@ its `Protocol` class); `@modelcontextprotocol/core` is a required peer that `cli
   `app.setRequestHandler("some/method", { params: SomeParamsSchema }, (params, ctx) => …)`
   for custom methods (the handler receives the parsed params); the two-argument
   `setRequestHandler("tools/call", handler)` form exists only for spec-defined
-  method names.
+  method names. The 1.x `(Schema, handler)` form still works on `App` and
+  `AppBridge` as a deprecated overload: it logs a one-time warning, hands the
+  handler the whole `{ method, params }` message as before, and gives request
+  handlers a 1.x-shaped `extra` (`signal`, `requestId`, `sessionId`, `_meta`).
+  It will be removed in 3.0.
 - **Errors.** Remote JSON-RPC errors are `ProtocolError` (numeric `code`);
   local conditions are `SdkError` with a string `code`: request timeout →
   `"REQUEST_TIMEOUT"`, connection closed → `"CONNECTION_CLOSED"`. Cancelling a