fix: reject contradictory running break sessions

tests/test_today_store.py

@@ -92,6 +92,14 @@ async def test_active_session_api_persists_a_bounded_break_deadline(monkeypatch,
     assert (await main.get_today_session()) == saved
 
 
+def test_active_session_api_rejects_a_running_break():
+    with pytest.raises(ValueError, match="break must remain paused"):
+        main.TodaySessionUpdate(
+            base_revision=0, device_id="phone-a", identity="issue:r:1:",
+            elapsed_ms=5_000, running=True, break_deadline_at=1_800_000,
+        )
+
+
 @pytest.mark.anyio
 async def test_active_session_api_returns_current_state_on_claim_conflict(monkeypatch, tmp_path):
     async def user():