Remove isCallToolResult and convertCallToolResult from public exports

These are internal implementation details used by session.ts and client.ts.
Go and Python already keep them private (lowercase/underscore-prefixed).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: stephentoub

nodejs/src/index.ts

@@ -13,8 +13,6 @@ export { CopilotSession, type AssistantMessageEvent } from "./session.js";
 export {
     defineTool,
     approveAll,
-    isCallToolResult,
-    convertCallToolResult,
     SYSTEM_PROMPT_SECTIONS,
 } from "./types.js";
 export type {