firmware: prevent RS485 echo starvation

tests/test_xiao_sensor_node_contract.py

@@ -56,6 +56,12 @@ def test_level_calibration_is_persistent_and_non_blocking():
     assert "updateLevelCalibration" in loop
     assert "delay(5000)" not in calibration
 
+def test_rs485_echo_cannot_starve_sensor_loop():
+    transport = source("rs485_transport.cpp")
+    assert "bytesRemaining" in transport
+    assert "discardCommandUntilNewline" in transport
+    assert 'type == "sensor_status"' in transport
+
 def test_node_does_not_take_cargo_or_ble_authority():
     combined = "\n".join(p.read_text() for p in NODE.glob("*.*"))
     assert "NimBLE" not in combined and "JBD" not in combined and "server.on(" not in combined