Requests one or more invoice adjustments (refunds) in a single call.

Each entry is evaluated independently. The response is 202 Accepted with
billingAdjustments for accepted entries and errors for rejected entries.
Match each result to its request entry by clientToken. A response with an
empty billingAdjustments array means every entry failed.

Accepted entries start at PENDING. Processing can take up to 72 hours.
Poll the list endpoint or subscribe to the agreement_billing_adjustment_updated
webhook for status updates.

clientToken is the idempotency key for each entry. Reusing a token returns
the original result instead of creating a duplicate.

Each adjustmentAmount is an unsigned magnitude (for example, "200.00").
It must not exceed the invoice's pricingCurrencyAmount.maxAdjustmentAmount,
and currencyCode must match the invoice currency. Amount or currency
violations appear as per-entry errors in the 202 response, not as a
request-level 400.