Updates metadata for multiple customers

Patches metadata on up to 1,000 customers in one atomic request. Each customer may receive different values. If a customer is missing or any key or value is invalid, nothing is written for any customer. Per customer this behaves exactly like patch-customer-metadata: keys omitted for that customer are left unchanged; null, empty, and whitespace-only values clear that key. Metadata-only changes are recorded in the audit log but do not trigger customer.updated webhooks or customer integration syncs.