Re-enable Core Tests

Parent Epic: #155

# Task: Re-enable Core Tests

## Description
Re-enable and fix all disabled tests in test_htfa_core.py.disabled, ensuring they pass with the new parameter implementation.

## Acceptance Criteria
- [ ] test_htfa_initialization passes
- [ ] test_htfa_different_levels passes
- [ ] test_htfa_reproducibility passes
- [ ] test_invalid_backend passes
- [ ] All parametrized tests pass
- [ ] No regression in existing tests

## Technical Details
- Rename test_htfa_core.py.disabled to test_htfa_core.py
- Fix each test to work with new implementation
- Add new tests for parameter combinations
- Ensure 90%+ test coverage maintained
- Run full test suite to verify no regressions

## Dependencies
- [ ] All implementation tasks (164, 157, 158, 159, 160, 161) must be complete

## Effort Estimate
- Size: M
- Hours: 6-8
- Parallel: false

## Definition of Done
- [ ] All disabled tests re-enabled
- [ ] All tests passing
- [ ] Coverage above 90%
- [ ] CI/CD pipeline green
- [ ] Test documentation updated