[Feature] Add escrow contract upgrade authority — allow contract admin to migrate state to a new contract version

[Dami24-hub]

Context

contracts/escrow/src/lib.rs has no upgrade mechanism. When the escrow logic needs to change (e.g., adding partial release), all existing escrows using the old contract address are frozen in the old version. Soroban supports env.deployer().update_current_contract_wasm() for in-place upgrades.

Technical Requirements

• Add DataKey::Admin and an admin: Address parameter to initialize
• Add pub fn upgrade(env: Env, admin: Address, new_wasm_hash: BytesN<32>) requiring admin.require_auth() and admin == stored_admin
• The upgrade should only be allowed when Released == false (funds still locked) — otherwise there is nothing to migrate
• Add tests: test_upgrade_by_admin_succeeds, test_upgrade_by_non_admin_panics, test_upgrade_after_release_panics

Definition of Done

• upgrade function exists in lib.rs
• DataKey::Admin is a new storage key
• ≥ 3 tests pass

[Guddy1-tech]

@Guddy1-tech has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I’d like to work on this issue because it matches my current learning path and will help me improve my software engineering skills. I have experience with Java, Python, JavaScript, React, APIs, and Git/GitHub, and I’m currently building real-world projects. I’m eager to contribute.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Guddy1-tech to this issue.

[olalois]

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

Hello Maintainer, I’ll love to this on, I’m exceptional with Soroban contracts and will implement a secure admin-gated upgrade flow with proper checks and tests to ensure safe state-preserving contract upgrades.

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