strategy: align model exit signal as assist-only trigger (#369)

tests/test_strategy_exit_rules.py

@@ -22,7 +22,7 @@ def test_take_profit_exit_for_backward_compatibility() -> None:
     assert out.reason == "arm_take_profit"
 
 
-def test_model_assist_exit_signal() -> None:
+def test_model_assist_signal_does_not_exit_directly() -> None:
     out = evaluate_exit(
         current_state=PositionState.ARMED,
         config=ExitRuleConfig(model_prob_threshold=0.62, arm_pct=10.0),
@@ -34,5 +34,5 @@ def test_model_assist_exit_signal() -> None:
             liquidity_weak=True,
         ),
     )
-    assert out.should_exit is True
-    assert out.reason == "model_liquidity_exit"
+    assert out.should_exit is False
+    assert out.reason == "hold"

tests/test_strategy_state_machine.py

@@ -28,3 +28,16 @@ def test_exited_has_priority_over_promotion() -> None:
         ),
     )
     assert state == PositionState.EXITED
+
+
+def test_model_signal_is_assist_only_not_direct_exit() -> None:
+    state = promote_state(
+        PositionState.ARMED,
+        StateTransitionInput(
+            unrealized_pnl_pct=1.0,
+            be_arm_pct=1.2,
+            arm_pct=2.8,
+            model_exit_signal=True,
+        ),
+    )
+    assert state == PositionState.ARMED