Add an invoice cancellation function allowing admins to cancel a registered invoice before it reaches funding target

[Chucks1093]

Summary

There is no mechanism to cancel an invoice that was registered in error or needs to be withdrawn before funding completes. A cancel_invoice function that transitions the invoice to Cancelled status and enables investor refunds should be added.

Scope

• Add cancel_invoice(invoice_id: BytesN<32>) callable only by the protocol admin
• Allow cancellation only when status is Open
• Transition the invoice status to Cancelled
• Panic with InvalidInvoiceStatus if the invoice is already Funded or Settled
• Emit an invoice_cancelled event with invoice_id and the cancelling admin address

Acceptance Criteria

• Status transitions to Cancelled on successful cancellation
• Cancellation of a Funded invoice panics with InvalidInvoiceStatus
• Cancellation of a Settled invoice panics with InvalidInvoiceStatus
• Non-admin caller panics with Unauthorized
• invoice_cancelled event emitted with correct fields

ETA: 24 hours

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Coordinate on Telegram

[startarget001]

@startarget001 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

This project aligns well with my experience. I'm confident in providing a clean, effective, and thoroughly tested solution.

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[The-Big-Danny]

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

hello maintainer, i'd love to work on this issue! please assign me

ℹ️ Repo Maintainers: To accept this application, review their application or assign @The-Big-Danny to this issue.

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