Add payment delegation — allow a third party to pay on behalf of a payer

[Kingsman-99]

Overview

Currently only the address initiating the transaction can be recorded as the payer. Add delegation so a sponsoring address can pay an invoice on behalf of another address (e.g. a company paying on behalf of an employee).

Acceptance Criteria

• pay_invoice_delegated(invoice_id, on_behalf_of: Address) function
• Caller provides funds; on_behalf_of address is recorded as the payer
• on_behalf_of must have signed a delegation authorization stored via set_delegation(invoice_id, delegate: Address)
• set_delegation callable only by the address granting the delegation
• Delegation is single-use — consumed on first pay_invoice_delegated call
• Payment history records both the payer (on_behalf_of) and executor (caller)
• Emits DelegatedPayment { invoice_id, payer, executor, amount, ledger } event
• Integration tests: delegation grant → delegated pay, replay attack prevention

[Killerjunior]

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Hi maintainer, I have read and understood the issue requirements and checked the necessary documentation. I’m confident I can work on this task effectively and deliver a proper solution.

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[brite-side0]

@brite-side0 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

hello dear maintainer i would love to work along side you on this task sir.

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[grantfox-oss]

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@brite-side0's PR #338 was approved and merged by @Kingsman-99.

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