Implement Invoice Due Date Bounds

[Baskarayelu]

Description
Enforce max due date (e.g. now + max_due_date_days from protocol config) on store_invoice and upload_invoice so due dates cannot be set arbitrarily far in the future.

Requirements and context

• Must be secure, tested, and documented
• Use protocol config or constants
• Smart contracts only (Soroban/Rust)

Suggested execution

1. Fork the repo and create a branch
   git checkout -b feature/invoice-due-date-bounds
2. Implement changes
   • Implement: validation in store_invoice/upload_invoice using init/protocol_limits
   • Add tests for max due date boundary
   • Document in docs/contracts/invoice.md
3. Test and commit
   • Run cargo test
   • Include test output and security notes

Example commit message
feat: invoice due date bounds with tests and docs

Guidelines

• Minimum 95 percent test coverage
• Clear documentation
• Timeframe: 96 hours

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[Kevin737866]

@Kevin737866 has applied to work on this issue as part of the Stellar Wave Program's 2nd wave.

I would love to work on this project, and it would be done correctly and before the deadline

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[Jeyvers]

@Jeyvers has applied to work on this issue as part of the Stellar Wave Program's 2nd wave.

Hi again, please I'd like to work on this and would do it properly.

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Congratulations, @Kevin737866! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on February 26, 2026.

🧑‍💻 @Kevin737866: 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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