fix: inconsistence in promotion schema

[vanpho93]

Impact: minor
Type: bugfix

Issue

• Incorrect discount value
• Inconsistence on promotion schema

Solution

• Update discount item calculation
• Change field names

Testing

Added new unit test file:

• packages/api-plugin-promotions-discounts/src/methods/index.test.js

[changeset-bot]

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Latest commit: 1a2189b

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