fix(esp-sync): get last payment amount from actual completed order

[dkoo]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Fixes a bug where the NP_Last Payment Amount value isn't always accurate. Currently we assume that if the subscription is active, the last payment must match its renewal price, but that's not always the case—for example, for new subscriptions that include a free trial period. In this scenario, the subscription in free trial mode will be active and have a successful completed parent order, but that parent order's amount will be 0 instead of the recurring amount.

Closes 1200550061930446/1209296877142332.

How to test the changes in this Pull Request:

1. On a site with RAS setup and connected, create a subscription product with a non-zero cost and a free trial period.
2. On release, purchase a subscription for that product as a new reader.
3. In the connected ESP or via logging, observe that the NP_Last Payment Amount is set to the cost of the subscription, even though the NP_Total Paid value is correctly 0.
4. Check out this branch and trigger a resync of this contact via any means.
5. Confirm that NP_Last Payment Amount is now 0 after the resync.
6. Trigger a manual early renewal of the subscription to charge the customer, and confirm that NP_Last Payment Amount is now the amount charged in the renewal order.
7. Smoke test purchasing a subscription product and/or resyncing a contact who has previously purchased a subscription WITHOUT a free trial period and confirm that the NP_Last Payment Amount is still accurate.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

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[naxoc]

Yup! I tested and smoke tested and it looks good.

[matticbot]

🎉 This PR is included in version 5.13.1 🎉

The release is available on GitHub release

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