UN-2813 [CLEANUP] Remove invalid Hatchet and circuit breaker tests

Remove test files that are not applicable or not working:

**Removed Files:**
- tests/contract/test_hatchet_adapter_contract.py
- tests/integration/test_hatchet_backend.py
- tests/integration/test_circuit_breaker.py

**Rationale:**
- Hatchet backend tests are not currently needed/working
- Circuit breaker tests are invalid for current implementation
- Cleanup reduces test suite complexity and removes failing tests

**Remaining Tests:**
- Contract tests: backend_contract, celery_adapter, temporal_adapter, migration_api (4)
- Integration tests: backend_selection, celery_backend, cross_backend_compatibility,
  end_to_end, feature_flag_rollout, service_replacement, temporal_backend (7)
- Unit tests: unchanged (4)

Total: 15 test files remaining (down from 18)

No broken imports or references - all remaining tests are independent.

Related: #1555

Author: Deepak-Kesavan