refactor(constants): enforce StatusEnum in tests — replace hardcoded status strings (#3533)

[mrveiss]

Summary

Replaces raw status string literals in co-located orchestration test files with TaskStatus enum constants from constants/status_enums.py.

Changes

• orchestration/dag_executor_test.py — 7 assertions ("completed", "failed") → TaskStatus.COMPLETED.value / TaskStatus.FAILED.value
• orchestration/variable_resolver_test.py — 12 occurrences (assertions, default arg values, constructor args) → enum constants
• orchestration/error_handler_test.py — 11 occurrences (constructor args, assertions) → enum constants

Why

When a status value is renamed in the enum, tests using raw strings silently pass but test the wrong value. Using TaskStatus.X.value breaks at import if the constant is removed.

Skipped

• conversation_file_manager_init_test.py:409 — checks a SQLite migration tuple column, not an orchestration status
• async_baseline.performance_test.py — "failed" is a local return value, not the StatusEnum domain
• orchestration/workflow_executor.py + enhanced_orchestration/workflow_planning.py — dict .get("status") comparisons against external JSON payloads (tracked separately in chore(backend): expand hardcoding prevention beyond IPs/ports — database URLs, timeouts, model names #3397)

Closes #3533

[github-actions]

✅ SSOT Configuration Compliance: Passing

🎉 No hardcoded values detected that have SSOT config equivalents!