Include Change Amount in Order Summary OpenAPI

Problem Statement

Currently, customers upgrading plans cannot see the exact price difference (including proration) before confirming the change.

Although the backend calculates this internally, the OpenAPI response for order summary does not expose this information, leaving the frontend unable to display accurate totals.

User Story

As a subscriber, I want to see a detailed breakdown of how my upgrade or plan change amount is calculated — including any prorated charges — so that I can make an informed decision before confirming the change.

Definition of Done (DoD)

1. Extend Subcription Summary OpenAPI ) to include:

2. Total difference charge > change_summary_total (field name should be similar to core API field)

3. Ensure returned totals match backend billing calculations used in the actual plan change process.

4. UI / SDK

5. Update Change Plan modal / component to:

6. Display the change difference amount in the order summary section.

7. Format amounts consistently with platform pricing (currency, sign).

8. Update OpenAPI docs to reflect new response structure.

9. Unit and integration tests confirming accuracy for:

10. Plan upgrade

11. Downgrade

12. Mid-cycle changes

13. Confirm SDK handles new fields without regression.

Affected Components

1. Backend

2. Default UI

3. Crain UI