feat(contracts): add contract upgradeability

[Itodo-S]

Summary

• Implement contract upgradeability using Soroban's built-in update_current_contract_wasm mechanism
• Admin-only upgrade(admin, new_wasm_hash) function replaces bytecode while preserving all persistent and instance storage
• Add version() query (returns 1) for tracking contract versions across upgrades

Changes

contracts/src/lib.rs:

• Import BytesN for WASM hash parameter
• Add upgrade() — validates admin auth, calls env.deployer().update_current_contract_wasm()
• Add version() — returns current contract version constant
• Add tests: test_version_returns_current and test_upgrade_rejects_non_admin

Upgrade flow

# 1. Build new contract version
cargo build --target wasm32-unknown-unknown --release

# 2. Install the new WASM on-chain
soroban contract install --wasm target/wasm32-unknown-unknown/release/agenticpay.wasm --network testnet

# 3. Call upgrade with the returned hash
soroban contract invoke --id $CONTRACT_ID --network testnet -- upgrade --admin $ADMIN_ADDRESS --new_wasm_hash $WASM_HASH

Test plan

• test_version_returns_current — version() returns 1
• test_upgrade_rejects_non_admin — non-admin upgrade panics with "Only admin can upgrade"

Closes #42

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[Smartdevs17]

fix merge confllicts