[CONTRACT] Add a mutual deadline extension entrypoint

[Cedarich]

Complexity: Medium (150)

Description
deadline is fixed at create_escrow and never changeable. When work legitimately runs late, the parties' only options today are to let refund_expired claw everything back to the depositor or to cancel and recreate the escrow — both of which destroy the milestone history and force re-funding. Add an entrypoint that lets depositor and recipient jointly move the deadline forward.

Acceptance Criteria

• A new entrypoint extends the deadline and requires auth from both depositor and recipient
• The new deadline must be strictly later than the current one and satisfy the same InvalidDeadline validation as creation
• Extension is rejected for terminal statuses (Completed, Cancelled, Resolved, Expired) and while the contract is paused, consistent with the existing guard pattern
• A versioned event is emitted with the old and new deadline
• Tests cover happy path, single-party auth rejection, earlier-deadline rejection, terminal-state rejection, and interaction with refund_expired

[captainjoel2019-bit]

@captainjoel2019-bit has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi Maintainer, I’ve gone through the issue details and understand the problem. I’d be happy to work on a solution.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @captainjoel2019-bit to this issue.

[omoh25]

@omoh25 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

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.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @omoh25 to this issue.

[escaprt]

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

hello can you give me one issue pls

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

[drips-wave]

Congratulations, @omoh25! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

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