feat: implement integration test suite (SC-83)

[diegoucampos-tech]

Pull Request

Description

Implementation of the cross-contract integration test suite for the TrustUp-Contracts project as described in SC-83. This includes testing the core BNPL lifecycle, default scenarios, unauthorized access handling, and fixing various runtime panics discovered during testing.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Changes Made

• Added a new tests/integration package to the workspace.
• Implemented TestEnv setup for initializing and linking CreditLine, LiquidityPool, MerchantRegistry, and Reputation contracts using soroban-sdk testutils.
• Added integration tests for:
  • Full BNPL lifecycle (test_full_bnpl_lifecycle).
  • Default loan scenarios (test_loan_default).
  • Unauthorized access (test_unauthorized_access).
• Fixed runtime panics in tests:
  • Initialized Reputation contract admin during setup.
  • Authorized CreditLine in LiquidityPool via set_creditline during setup.
  • Adjusted loan requested amounts to prevent ExposureLimitExceeded panics.
  • Fixed repay_loan amount logic in tests to accurately reflect remaining balance, preventing InvalidRepaymentAmount errors.
• Fixed reputation-contract unit tests by correctly filtering SCORECHGD events by reason before asserting values.
• Removed Cargo.lock from .gitignore and committed it to ensure reproducible builds in CI and prevent soroban-env-host trait bound compilation errors.

Testing

• Tests pass locally
• New tests added (if applicable)

Checklist

• Code follows project style guidelines
• Self-review completed
• Comments added for complex code
• Documentation updated
• No new warnings generated

Closes #83