Add tests for `submit_invoices_batch` with mixed valid/invalid

[Levi-Ojukwu]

Description:

The batch submission function needs tests for edge cases.

Why it matters: Batch operations have complex validation logic that needs testing.

Acceptance Criteria:

• Test batch with all valid invoices
• Test batch rejection when size > 10
• Test batch with some invalid invoices (should fail entirely)
• Test referral tracking across batch

Relevant Files: contracts/invoice_liquidity/src/tests_new_features.rs

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[cristianooR7]

@cristianooR7 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

hello maintainer, i'd lik to work on this

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[DeFiVC]

@DeFiVC has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

I would love to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @DeFiVC to this issue.

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Congratulations, @DeFiVC! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

🧑‍💻 @DeFiVC: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@DeFiVC's PR #506 was approved and merged by @Levi-Ojukwu.

🏆 @DeFiVC: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (452 total points). Track your full progress on GrantFox.

👏 Great work, @DeFiVC! Keep contributing to Invoice-Liquidity-Network.

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This issue has been completed by @DeFiVC as part of the Stellar Wave Program's 7th Wave 🥳

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