refactor(workflow): delegate start_execution to TestEntity

Replace manual state+field mutation with entity.start_execution() and apply_to(), keeping audit logging and notifications at the service layer.

backend/app/services/test_workflow_service.py

@@ -111,15 +111,33 @@ def start_execution(db: Session, test: Test, user: User) -> Test:
     """Move from ``draft`` → ``red_executing``.
 
     Typically called by a **red_tech** when they begin the attack.
-    Starts the Red Team timer by recording ``red_started_at``.
+    Delegates to :meth:`TestEntity.start_execution` which handles the
+    state transition and sets ``execution_date`` / ``red_started_at``.
     """
-    now = datetime.utcnow()
-    test = transition_state(
-        db, test, TestState.red_executing, user,
-        action_name="start_execution",
+    entity = TestEntity.from_orm(test)
+    entity.start_execution()
+    entity.apply_to(test)
+    db.flush()
+
+    log_action(
+        db,
+        user_id=user.id,
+        action="start_execution",
+        entity_type="test",
+        entity_id=test.id,
+        details={
+            "previous_state": "draft",
+            "new_state": test.state.value,
+            "test_name": test.name,
+            "technique_id": str(test.technique_id),
+        },
     )
-    test.execution_date = now
-    test.red_started_at = now
+
+    try:
+        notify_test_state_change(db, test, test.state.value)
+    except Exception as e:
+        logger.warning("Notification failed for test %s: %s", test.id, e, exc_info=True)
+
     return test