feat: implement contract upgradeability API (Closes #125)

[Abrahamojobo]

Closes #125

Description

This PR integrates the standard Soroban mutable upgrade execution pipelines into the MarketXContract. It unlocks the ability for the Admin to hot-swap bug fixes and patch enhancements directly onto the deployed contract map without migrating the underlying ledger.

Changes Included

• Upgrade Trait Integration (lib.rs):
  • Added the upgrade endpoint that safely executes the native env.deployer().update_current_contract_wasm(new_wasm_hash) vector.
  • Placed an overarching Self::assert_admin(&env)? boundary block, explicitly trapping any execution requests from end-users, fee collectors, or unauthorized arbiters.
• Deep Native Tests (test.rs):
  • test_upgrade_auth_failure: Validates authentication blocks by spinning up a mock upgrade environment and asserting that external random non-admin invokers fail cleanly with an is_err() catch.
  • test_upgrade_state_persistence: Simulates a fully funded Escrow node and executes a live test dummy hash upgrade across the test ledger, subsequently re-querying the exact escrow_id node mapping to mathematically assert escrow.amount == 1000 is preserved linearly across the environment transaction line unharmed.

Review Guide

Reviewers should specifically verify the upgrade logic inside the Admin Block in contracts/marketx/src/lib.rs.