[5] Add `upgrade()` function to all 5 contracts

[EmeditWeb]

Problem

None of the 5 contracts (creditline, liquidity-pool, reputation, vendor-registry, token-mock) currently expose an upgrade() entry point. Once deployed to testnet, any bug fix requires a full redeploy and address change — breaking every client that references the old address.

Context

Soroban supports in-place WASM upgrades preserving contract address and storage. Without this, every contract bugfix invalidates the API's contract IDs and the mobile app's hardcoded references. Upgradeability is the difference between a one-shot deploy and a maintainable protocol.

Before Starting

Read these context files first:

• context/architecture-context.md
• context/code-standards.md
• context/progress-tracker.md
• contracts/*/src/lib.rs

What To Build

1. Define a shared snippet (in each contract's lib.rs):

   pub fn upgrade(env: Env, new_wasm_hash: BytesN<32>) -> Result<(), ContractError> {
       let admin: Address = env.storage().instance().get(&StorageKey::Admin).ok_or(ContractError::NotInitialized)?;
       admin.require_auth();
       env.deployer().update_current_contract_wasm(new_wasm_hash);
       Ok(())
   }

2. Add to all 5 contracts: creditline, liquidity-pool, reputation, vendor-registry, token-mock.
3. Emit a CONTRACTUPGRADED event with (old_version, new_version, ts) — add a get_version() -> u32 function returning current version, bumped by upgrade.
4. Persist StorageKey::Version in each contract's instance storage, default 1.
5. Write one unit test per contract: non-admin call rejected, admin call succeeds (mock the wasm hash).
6. Document the upgrade flow in contracts/README.md.

Files To Touch

• contracts/creditline-contract/src/lib.rs
• contracts/liquidity-pool-contract/src/lib.rs
• contracts/reputation-contract/src/lib.rs
• contracts/vendor-registry-contract/src/lib.rs
• contracts/token-mock-contract/src/lib.rs
• Each contract's events.rs, storage.rs, errors.rs
• contracts/README.md

Acceptance Criteria

• All 5 contracts expose upgrade(env, new_wasm_hash)
• All 5 contracts expose get_version() returning a u32
• Admin auth check is the FIRST executable line of upgrade()
• 5 new tests pass (one per contract)
• CONTRACTUPGRADED event emitted on successful upgrade
• Upgrade flow documented in README

Mandatory Checks Before PR

• cargo build passes with zero errors
• cargo test — all 93 existing tests still pass
• require_auth() is FIRST line of every mutating function
• extend_ttl() called after EVERY persistent storage write
• New unit tests written for every new function
• context/progress-tracker.md updated

[adelekevictor12]

I WILL LIKE TO WORK ON THIS ISSUE

[grantfox-oss]

🦊 GrantFox — @adelekevictor12 has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #5)
2. Your PR will be reviewed by the StepFi-app maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@adelekevictor12's PR #30 was approved and merged by @EmeditWeb.

🏆 @adelekevictor12: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (48 total points). Track your full progress on GrantFox.

👏 Great work, @adelekevictor12! Keep contributing to StepFi-app.