Add an invoice extension function allowing admins to push back the funding deadline for an open invoice

[Chucks1093]

Summary

Some invoices may need more time to reach their funding target. An extend_deadline function that updates the deadline_ledger for an open invoice would allow admins to give struggling invoices additional time without cancelling and re-registering them.

Scope

• Add extend_deadline(invoice_id: BytesN<32>, new_deadline_ledger: u32) callable only by the protocol admin
• Validate the invoice status is Open and new_deadline_ledger is greater than the current deadline_ledger
• Update deadline_ledger in persistent storage with a TTL bump
• Panic with DeadlineNotExtended if new_deadline_ledger is not greater than the current value
• Emit a deadline_extended event with invoice_id, old_deadline_ledger, and new_deadline_ledger

Acceptance Criteria

• deadline_ledger updated correctly for valid input
• Extension on a non-Open invoice panics with InvalidInvoiceStatus
• new_deadline_ledger not greater than current panics with DeadlineNotExtended
• Non-admin caller panics with Unauthorized
• deadline_extended event emitted with old and new values

ETA: 24 hours

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[martinifeanyi058-ship-it]

@martinifeanyi058-ship-it has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello Maintainer, Please i would love to work on this issue as it's something i can resolve.

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[iamcracked-dev]

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

Hi maintainers!
I’d love to help out with this issue. I’ve checked the current codebase/documentation and know exactly what needs to be updated to make this clearer. I will ensure everything aligns perfectly with your existing contribution guidelines.

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