feat: post a journal entry for cash-up variances

[dvdl16]

Description

• feat: post a journal entry for cash-up variances
  • Internal ref: TASK-2026-00126
  • Fixes the closing entry for POS does not post closing amount difference to write-off amount  frappe/erpnext#43392

Type of change

• Bug fix (change which fixes an issue)
• New feature (change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Tests

• Unit Tests have been updated or added, as required
• UI Tests have been updated or added, as required

Checklist:

• My code follows Naming Guidelines (DocType, Field and Variable naming)
• No Form changes / My code follows the Form Design Guidelines
• My code follows the Coding Standards of this project
• My code follows the Code Security Guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas / No comments necessary
• I have made corresponding additions/changes to the documentation
• All business logic and validations are on the server-side / No business logic or validation changes
• No patches are necessary / Migration Patches have been added to the correct subdirectory of /patches and patches.txt have been updated

User Experience: