Add ACH stop-payment support

[claude]

Requested by Julia Park · Slack thread

What

Adds ACH stop-payment support to the SDK, mirroring the existing check stop-payment support:

• CreateAchStopPaymentRequest (JSON:API type achStopPayment) in unit/models/payment.py.
• UpdateAchStopPaymentRequest (JSON:API type achStopPayment) for the update endpoint, also in unit/models/payment.py.
• A new ach_stop_payments resource exposing create and update (plus list/disable), wired onto the Unit facade.

The SDK previously supported only CHECK stop payments; this fills in the ACH variant.

Why

Truss needs to create and update ad-hoc ACH stop payments in Unit to block unauthorized third-party debits for a customer. The existing SDK only covered check stop payments, leaving no way to call the ACH stop-payment endpoints.

How

• New request model CreateAchStopPaymentRequest(UnitRequest) with constructor (originator_name=None, direction="Debit", min_amount=None, expiration=None, is_multi_use=False, description=None, relationships=None). to_json_api() delegates to UnitRequest.to_payload("achStopPayment", self.relationships), which camelCases attribute keys (originatorName, minAmount, isMultiUse, etc.) and attaches the account relationship.
• New request model UpdateAchStopPaymentRequest(UnitRequest) with constructor (stop_payment_id, tags=None). stop_payment_id is used only in the URL (PATCH /stop-payments/{id}) and is kept out of the JSON:API body; to_json_api() emits type: "achStopPayment" with the updatable tags attribute, mirroring the existing Patch*PaymentRequest models.
• New AchStopPaymentResource(BaseResource) in unit/api/ach_stop_payment_resource.py with self.resource = "stop-payments" and a create(request) that POSTs and decodes the response via DtoDecoder (falling back to RawUnitObject, same as the check version). Also includes update(request) (PATCH /stop-payments/{id}), list() (GET /stop-payments) and disable(stop_payment_id) (POST /stop-payments/{id}/disable).
• Wired self.ach_stop_payments = AchStopPaymentResource(api_url, token) into the Unit facade in unit/__init__.py, next to check_stop_payments.

Public interface (called by the Truss API):

from unit.models.payment import CreateAchStopPaymentRequest, UpdateAchStopPaymentRequest
unit_client.ach_stop_payments.create(request)
unit_client.ach_stop_payments.update(request)

Dependency note

The consuming Truss API PR depends on this SDK change being merged/released first.

Refs ENG-4839.

Consumed by Truss-pmts/api#5019 (https://github.com/Truss-pmts/api/pull/5019).

[samuelvasco]

question: should we also add a get method?

[claude]

Good call — added a get(stop_payment_id) method in f220f19.