Updates metadata for multiple bills

Patches metadata on up to 1,000 bills in one atomic request. Each bill may receive different values. If a bill is missing or any key or value is invalid, nothing is written for any bill. Per bill this behaves exactly like patch-bill-metadata: keys omitted for that bill are left unchanged; null, empty, and whitespace-only values clear that key. If any bill has a pending approval request, the entire request is rejected with a 400 and nothing is written.