core: implement vendor verification and approval flow

[EmeditWeb]

Problem

register_vendor() immediately marks vendors as active.
There is no approval process. Any address can register
and immediately start receiving loan disbursements.

What To Build

1. Add VendorStatus enum (Pending, Approved, Suspended, Rejected)
2. register_vendor() sets status = Pending
3. Add approve_vendor(), suspend_vendor()
4. Verify vendor status in create_loan()

Files To Touch

• contracts/vendor-registry-contract/src/lib.rs
• contracts/vendor-registry-contract/src/types.rs
• contracts/creditline-contract/src/lib.rs
• contracts/creditline-contract/src/errors.rs

Acceptance Criteria

• New vendors start as Pending
• Only Approved vendors can receive loans
• Admin approval flow works
• Suspension prevents new loans not old ones

Mandatory Checks Before PR

• cargo build passes
• All tests pass
• PR references this issue

[Adeyemi-cmd]

hello maintainer. i would love to work on this