fix(core): Address duplicate RefundLine issue when issuing refund across multiple payments

[HouseinIsProgramming]

Description

fixes #4135

this PR includes a failing test that should NOT be failing.
PR will be expanded with fixes needed to have this test pass as expected.

Checklist

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• I have added or updated test cases
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Project	Deployment	Review	Updated (UTC)
docs	Ready	Preview, Comment	Jan 19, 2026 3:13pm
vendure-storybook	Ready	Preview, Comment	Jan 19, 2026 3:13pm

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[sonarqubecloud]

Quality Gate failed

Failed conditions
C Security Rating on New Code (required ≥ A)

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