feat(core): share API request assembly between both transports

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/core/src/index.ts

@@ -17,6 +17,8 @@ export {
   requireRequestContext,
 } from "./request-context.ts";
 export { createExecutionContext, executionContextFromHttp } from "./execution-context.ts";
+export { buildApiRequest } from "./api-request.ts";
+export type { BuiltApiRequest } from "./api-request.ts";
 export type {
   ExecutionContext,
   ExecutionContextInput,