Refactor contract testing into unified integration test framework

[Smartdevs17]

Context

Contract tests are scattered across multiple directories.

Current Limitation

Tests are in multiple locations with no unified framework.

Expected Outcome

• Unified test framework
• Test fixture management
• Test data seeding
• Test coverage reporting

Acceptance Criteria

• Unified test framework
• Test fixture management
• Test data seeding
• Test coverage reporting
• Test performance benchmarks
• Test documentation
• Test CI integration

Technical Scope

• stellar-lend/contracts/test-utils/src/ - Test utilities
• stellar-lend/contracts/common/src/ - Common test utils
• tests/e2e/ - E2E tests
• .github/workflows/ - Test CI