[CONTRACT] Add tests for InvalidAmount error variant across operations

[dinahmaccodes]

Tier: Medium

Description: Add comprehensive tests for invalid amount handling and validate the InvalidAmount error.

Tasks:

• Test zero amount escrow deposits
• Test negative amount operations (if possible)
• Verify InvalidAmount error (code 14)

Acceptance Criteria:

• Invalid amounts are rejected
• Error variant is returned correctly
• Valid amounts are accepted

Files: contracts/shipment/src/test.rs, contracts/shipment/src/test_escrow_arithmetic.rs

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[yangqingqu]

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

Hello maintainer, I would love to work on this issue. Kindly assign to me, thanks.

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[Mercy017]

@Mercy017 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

I've shipped over 380 contributions across the ecosystem/platform with a clean review history, also contributed to this project last wave — no abandoned PRs, no back-and-forth delays. I'd like to add this issue to that list.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Mercy017 to this issue.

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Congratulations, @Mercy017! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

🧑‍💻 @Mercy017: 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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[drips-wave]

This issue has been completed by @Mercy017 as part of the Stellar Wave Program's 7th Wave 🥳

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