chore: updated mock accounting service function names and comments

[vicerace]

Checklist

• [x] I have read the coding guide.
• [ ] My change requires a documentation update, and I have done it.
• [ ] I have added tests to cover my changes.
• [ ] I have filled out the description and linked the related issues.

Description

This change updates the accounting mock service to rename Debit/Credit related functions to PrepareDebit/PrepareCredit variants.

The corresponding comments were also updated to reflect these new function names, improving code clarity and consistency with the actual implementation.

Open API Spec Version Changes (if applicable)

Motivation and Context (Optional)

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[vicerace]

Please fix linter: subject must not be sentence-case, start-case, pascal-case, upper-case [subject-case]

@martinconic Thanks. Modified.