Add invoice bulk import validator with row-level errors

[Kingsman-99]

Label: complexity: high
Points: 200

Description

For creators bulk-creating invoices from a CSV/JSON import, validate every row up front and report all errors at once (not fail-fast on the first bad row), so the creator can fix everything in one pass.

Technical Context

New src/bulkImportValidator.ts. Validates an array of CreateInvoiceParams-like rows against the same constraints the contract enforces (positive amounts, recipients/amounts length match, valid deadline), independent of contract submission.

Acceptance Criteria

• validateBulkImport(rows) returns { validRows: number[], errors: { row: number, field: string, message: string }[] }
• Continues validating all rows even after finding errors in earlier ones
• Mirrors the exact validation rules enforced in the contract's _create_invoice_inner (positive amounts, length match, future deadline)
• Tests: all-valid set, all-invalid set, mixed set with multiple distinct error rows

---

[DatboiCaleb]

@DatboiCaleb has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi Good day Maintainer

I’m a smart contract and full-stack developer who turns complex ideas into secure, production-ready blockchain code. With strong skills in Rust and Solidity, I build systems that are fast, safe, and built to last.
My experience with the Stellar SDK and Soroban lets me ship real-world Web3 solutions that actually move value.

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

[drips-wave]

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

🧑‍💻 @DatboiCaleb: 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.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊

[grantfox-oss]

🦊 GrantFox — @DatboiCaleb has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #229)
2. Your PR will be reviewed by the Stellar-split maintainers

Good luck! Track your progress on GrantFox.