Switch tsconfig to nodenext (blocked by @modelcontextprotocol/ext-apps)

[harijoe]

Context

#673 enabled biome's useImportExtensions lint rule to enforce .js suffixes on relative imports in packages/core. This prevents ESM runtime crashes (ERR_MODULE_NOT_FOUND) when a developer forgets the extension.

However, the ideal fix is switching moduleResolution from bundler to nodenext in packages/core/tsconfig.json, so tsc itself rejects missing extensions at compile time rather than relying on a lint rule.

Problem

@modelcontextprotocol/ext-apps@1.3.2 ships .d.ts files with extensionless specifiers:

// dist/src/app.d.ts
export { PostMessageTransport } from "./message-transport";
export * from "./types";

Under nodenext, TypeScript can't resolve these, causing errors like:

• TS2305: Module '"@modelcontextprotocol/ext-apps"' has no exported member 'McpUiResourceMeta'
• TS2460: Module declares 'McpUiHostContext' locally, but it is exported as 'PostMessageTransport'

Next steps

• File an issue or PR upstream on modelcontextprotocol/ext-apps to add .js extensions in their declaration files
• Once fixed upstream, switch packages/core/tsconfig.json to module: "nodenext" / moduleResolution: "nodenext" and remove the biome project domain workaround