Add tests for multi-developer checkout payout correctness

[simonhamp]

Summary

• Adds feature tests verifying that checkouts containing plugins from multiple developers correctly assign proportional payouts to each developer
• Tests cover individual plugin carts, bundle carts with proportional allocation, mixed carts (individual + bundle), and the 15-day holding period enforcement
• Confirms eligible_for_payout_at is always set no less than 14 days after purchase

Test plan

• All 6 new tests pass (48 assertions)
• Pint formatting applied
• Run full test suite to check for regressions

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