✨ Description of Change

A concise summary of what is being changed. Please provide enough context for reviewers to be able to understand the change and why it is necessary. If the issue/ticket already provides enough information, you can put "See ticket" as the description.

Link to GitHub issue or Jira ticket: https://pihemr.atlassian.net/browse/OBPIH-6611

Description: Ensure that we don't include inverse prepayment invoice items in the amount outstanding report. See ticket for details.

📷 Screenshots & Recordings (optional)

If this PR contains a UI change, consider adding one or more screenshots here or link to a screen recording to help reviewers visualize the change. Otherwise, you can remove this section.

📈 Test Plan

We require that all code changes come paired with a method of testing them. Please select which of the following testing approaches you've included with this change:

• Frontend automation tests (unit)
• Backend automation tests (unit, API, smoke)
• End-to-end tests (Playwright)
• Manual tests (please describe below)
• Not Applicable

Description of test plan (if applicable):

• Create a PO with two items.
• Generate a prepayment invoice for the PO.
• Ship one item of the PO.
• Generate an invoice for the shipped item.
• Download an Amount Outstanding Report and verify that the report only contains the second, non-shipped/invoiced item and does not contain any rows relating to the inverse of the prepayment for any items.

✅ Quality Checks

Please confirm and check each of the following to ensure that your change conforms to the coding standards of the project:

• The pull request title is prefixed with the issue/ticket number (Ex: [OBS-123] for Jira, [#0000] for GitHub, or [OBS-123, OBPIH-123] if there are multiple), or with [N/A] if not applicable
• The pull request description has enough information for someone without context to be able to understand the change and why it is needed
• The change has tests that prove the issue is fixed / the feature works as intended