🪲 BUG-#205: Replace Execution with TaskEngine in Parallel strategy

[FernandoCelmer]

Description

Replace the legacy Execution class with TaskEngine in the Parallel strategy, and remove Execution entirely from the codebase.

Issue: 📌 ISSUE-#205

Changes

• Add _run_task static method to Parallel using TaskEngine directly
• Remove dotflow/core/execution.py (no longer used anywhere)
• Migrate test_execution.py tests (already covered by test_engine.py)
• Rename test_execution_result.py → test_engine_result.py
• Rename test_execution_orderer.py → test_engine_orderer.py
• Both migrated to use TaskEngine

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes