Add tests for the lock-time-limit guard

[bigpoppaofny]

Close: #2104

Pull Request Template

📝 Description

Here's a summary of changes:
New file: quicklendx-contracts/src/test_lock_time_limit_guard.rs
6 tests pinning the ProtocolLimitsContract::validate_invoice time window with the "Within, at limit, past" pattern:
Zone Lower bound Upper bound
Within due_date > current_time → OK due_date < max_due_date → OK
At limit due_date == current_time → Err due_date == max_due_date → OK
Past due_date < current_time → Err due_date > max_due_date → Err
Edited: quicklendx-contracts/src/lib.rs (line 140-141)
Added #[cfg(test)] mod test_lock_time_limit_guard; — runs on every CI matrix entry, not gated behind legacy-tests.
The environment lacks a native linker (link.exe) and network access, so cargo test and cargo clippy couldn't execute. The code matches existing patterns in test_due_date_guard.rs and test_protocol_limits_boundary.rs, and all six test cases were verified correct by manual trace through the validate_invoice function.

🎯 Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Refactoring
• Performance improvement
• Security enhancement
• Other (please describe):

🔧 Changes Made

Files Modified

New Files Added

Key Changes

🧪 Testing

• Unit tests pass
• Integration tests pass
• Manual testing completed
• No breaking changes introduced
• Cross-platform compatibility verified
• Edge cases tested

Test Coverage

📋 Contract-Specific Checks

• Soroban contract builds successfully
• WASM compilation works
• Gas usage optimized
• Security considerations reviewed
• Events properly emitted
• Contract functions tested
• Error handling implemented
• Access control verified

Contract Testing Details

📋 Review Checklist

• Code follows project style guidelines
• Documentation updated if needed
• No sensitive data exposed
• Error handling implemented
• Edge cases considered
• Code is self-documenting
• No hardcoded values
• Proper logging implemented

🔍 Code Quality

• Clippy warnings addressed
• Code formatting follows rustfmt standards
• No unused imports or variables
• Functions are properly documented
• Complex logic is commented

🚀 Performance & Security

• Gas optimization reviewed
• No potential security vulnerabilities
• Input validation implemented
• Access controls properly configured
• No sensitive information in logs

📚 Documentation

• README updated if needed
• Code comments added for complex logic
• API documentation updated
• Changelog updated (if applicable)

🔗 Related Issues

Closes #
Fixes #
Related to #

📋 Additional Notes

🧪 How to Test

📸 Screenshots (if applicable)

⚠️ Breaking Changes

🔄 Migration Steps (if applicable)

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📋 Reviewer Checklist

Code Review

• Code is readable and well-structured
• Logic is correct and efficient
• Error handling is appropriate
• Security considerations addressed
• Performance impact assessed

Contract Review

• Contract logic is sound
• Gas usage is reasonable
• Events are properly emitted
• Access controls are correct
• Edge cases are handled

Documentation Review

• Code is self-documenting
• Comments explain complex logic
• README updates are clear
• API changes are documented

Testing Review

• Tests cover new functionality
• Tests are meaningful and pass
• Edge cases are tested
• Integration tests work correctly

Closes #2104

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[bigpoppaofny]

Done, Close: #2381