feat: implement is_valid_loan() with tests

[emarc99]

Description

Implented the is_valid_loan functionality based on stated requirements.

Test Coverage Explanation:

1. Positive Case:

   • test_is_valid_loan_positive: Tests a properly initialized loan with ONGOING status

2. Negative Cases:

   • test_is_valid_loan_non_existent: Tests with a loan ID that doesn't exist

   • test_is_valid_loan_zero_id: Tests with loan_id = 0

   • test_is_valid_loan_mismatched_id: Tests when storage key doesn't match loan.id

   • test_is_valid_loan_uninitialized: Tests NOT_INITIALIZED status

3. Edge Cases:

   • test_is_valid_loan_repaid_status: Validates that REPAID loans are still considered valid

   • test_is_valid_loan_foreclosed_status: Validates that FORECLOSED loans are still considered valid

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Configuration change

Checklist

Development Best Practices

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• New and existing tests pass locally with my changes

Testing

• Unit tests cover the new functionality
• Integration tests verify the changes work with other components

Related Issues

• Closes: Implement is_valid_loan Functionality #23

Screenshots / Logs (if applicable)

Additional Context

[emarc99]

@jaykayudo, kindly review and let me know if any adjustments are needed. Thanks!

[jaykayudo]

LGTM! @emarc99
Nice job and Thank you