The package client has several download mirrors and needs a predictable fallback decision when some are unhealthy. Implement the selection rules so priority and region preferences are applied consistently.
Build the implementation in Go under /app, with /app/run.sh as its entry point. Read one JSON request from standard input and write one JSON result to standard output. Input has preferred_region, a requested package list, and mirrors in priority order with unique name, region, healthy boolean and packages list. For each request choose the first healthy mirror containing it in the preferred region, otherwise the first healthy containing mirror anywhere. Preserve repeated requests and return mirror names or null. Duplicate mirror names must be rejected.
Reject inputs that violate an explicit validation rule with exit status 2, no standard output, and {"error":"invalid_input"} on standard error. For valid input, exit with status 0. Keep the implementation reusable for new inputs; it must work offline and must not depend on the example data being the only data it receives.
Repair or replace the unfinished /app/run.sh and add source files under /app. Malformed JSON must use the same invalid-input response. Do not read /tests or /oracle from the application.