fix: add explicit .js extensions to relative imports so declarations resolve under NodeNext/Node16

The emitted .d.ts copy relative specifiers verbatim, and the extensionless form
(e.g. `from "./events"`) doesn't resolve under Node16/NodeNext, hiding inherited
members such as `addEventListener` (TS2339). Appending `.js` matches the existing
SDK-import style and works with both bundler-mode type-checking and Bun.build.
Types-only; runtime is unaffected.

Fixes #704

src/server/index.test.ts

@@ -6,7 +6,7 @@ import {
   RESOURCE_MIME_TYPE,
   getUiCapability,
   EXTENSION_ID,
-} from "./index";
+} from "./index.js";
 import type { McpServer } from "@modelcontextprotocol/server";
 
 describe("registerAppTool", () => {