Feat/issue 54 blacklist

[rayeberechi]

Overview

This PR implements a blacklisting mechanism to permanently ban addresses from the QuorumCredit protocol. This is a critical security feature to prevent repeat defaulters from accessing new loans.

Changes

• New Storage Key: Added DataKey::Blacklisted(Address) to persistent storage to track banned borrowers.
• Admin Function:
  • Implemented pub fn blacklist(env: Env, admin_signers: Vec<Address>, borrower: Address).
  • Secured with the existing multi-sig require_admin_approval logic.
• Borrowing Restriction:
  • Modified request_loan to include an early-exit check.
  • If a borrower is blacklisted, the contract now returns Err(ContractError::Blacklisted).
• View Function: Added is_blacklisted(borrower) -> bool to allow off-chain interfaces and users to verify the status of an address.
• Error Handling: Introduced ContractError::Blacklisted = 14.

Unit Tests

• test_is_blacklisted_defaults_to_false: Ensures new addresses are not banned by default.
• test_blacklist_prevents_loan_request: Confirms that once an admin blacklists a borrower, all subsequent try_request_loan calls fail with the correct error.
• test_non_admin_cannot_blacklist: Verifies that unauthorized addresses cannot trigger a blacklist entry.

Build Note

During development, it was confirmed that the repository has 56 pre-existing compile errors (largely related to legacy string encoding and duplicate definitions). A git stash verification confirmed that this PR introduces zero new errors and follows the established project architecture.

Closes #54

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