fix: refactor refund core

## Description
The division of the refund core into modular functions isn't in place, whilst the functionalities that are being called are used in multiple places for different use cases. The refactoring of the refund core is necessary.

## Changes
- In the refactoring the core execution logic of the refund must be separated from the `schedule_refund_execution` function
- Function for executing instant refund and scheduled refund must be separated

Start by locating schedule_refund_execution and the refund functionality it calls. Separate the core execution logic from scheduling, with distinct paths for instant and scheduled refunds, then verify both use cases retain their existing behavior.

Written by the indexing model from the issue text.