Add `get_invoice_funding_percentage` view function returning basis points

[Kingsman-99]

Description

A common UI need is "how much of this invoice is funded?" expressed as a percentage. Adding get_invoice_funding_percentage(invoice_id: u64) -> u32 that returns basis points (funded * 10_000 / total) avoids client-side division and surface-area bugs when total is zero.

Acceptance Criteria

• Add pub fn get_invoice_funding_percentage(env: Env, invoice_id: u64) -> u32 to contracts/split/src/lib.rs
• Returns (funded * 10_000 / total) as u32; returns 0 when total == 0
• Returns ContractError::InvoiceNotFound for unknown IDs
• Unit test covers 0%, 50%, and 100% cases
• cargo test passes

Context

• Target file: contracts/split/src/lib.rs
• Should use InvoiceHot fields where available to keep reads cheap

[tyson-mill]

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[chukwudiikeh]

@chukwudiikeh has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello i would like to work on this issue

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[grantfox-oss]

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

@chukwudiikeh's PR #646 was approved and merged by @Kingsman-99.

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