fix: make advance_journey idempotent for same-state transitions

Clicking action buttons multiple times (or page reruns) no longer
throws ValueError when already at the target journey state.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/services/state_manager.py

@@ -35,7 +35,8 @@ def get_current_journey_state() -> str:
 def advance_journey(target_state: str) -> None:
     """Advance the journey to *target_state*.
 
-    Raises ValueError if the target is invalid, the same as current, or backward.
+    No-op if already at *target_state* (idempotent).
+    Raises ValueError if the target is invalid or backward.
     """
     if target_state not in JOURNEY_STATES:
         raise ValueError(f"Invalid journey state: {target_state}")
@@ -45,7 +46,7 @@ def advance_journey(target_state: str) -> None:
     target_idx = JOURNEY_STATES.index(target_state)
 
     if target_idx == current_idx:
-        raise ValueError(f"Already at same state: {target_state}")
+        return  # idempotent — already at target state
     if target_idx < current_idx:
         raise ValueError(f"Cannot move backward from {current} to {target_state}")
 
@@ -80,3 +81,9 @@ def can_advance_to(target_state: str) -> bool:
             return False
 
     return True
+
+
+def reset_session_state() -> None:
+    """Reset all session state to defaults for a new session."""
+    for key, default_value in _DEFAULTS.items():
+        st.session_state[key] = default_value

app/tests/test_state_manager.py

@@ -88,10 +88,10 @@ def test_advance_refuses_backward(mock_session_state):
     assert mock_session_state["journey_state"] == "VALIDATE_TRIALS"
 
 
-def test_advance_refuses_same_state(mock_session_state):
+def test_advance_same_state_is_idempotent(mock_session_state):
     mock_session_state["journey_state"] = "PRESCREEN"
-    with pytest.raises(ValueError, match="same"):
-        advance_journey("PRESCREEN")
+    advance_journey("PRESCREEN")  # should not raise
+    assert mock_session_state["journey_state"] == "PRESCREEN"
 
 
 def test_advance_invalid_state(mock_session_state):