Include Change Amount in Order Summary OpenAPI

🔍 Problem Statement
As a subscriber upgrading a plan, I experience not seeing the exact prorated price difference before confirming a plan change, which reduces confidence and increases abandon rates.

💡 User Story
As a subscriber, I want the plan change order summary to display the exact amount due (with proration), so that I know exactly what I will be charged before confirming.

🎯 Definition of Done
- Given a plan change, when the order summary is displayed, then the prorated change amount is shown alongside the new recurring price.
- This change impacts the API (order summary response — expose changeAmount field) and JS-SDK/Elements (order summary component).
- Limitations: proration logic is unchanged; this exposes an existing calculated field only.

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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) Extend Subcription Summary OpenAPI ) to include: Total difference charge > change_summary_total (field name should be similar to core API field) Ensure returned totals match backend billing calculations used in the actual plan change process. UI / SDK Update Change Plan modal / component to: Display the change difference amount in the order summary section. Format amounts consistently with platform pricing (currency, sign). Update OpenAPI docs to reflect new response structure. Unit and integration tests confirming accuracy for: Plan upgrade Downgrade Mid-cycle changes Confirm SDK handles new fields without regression. Affected Components Backend Default UI Crain UI