Keep protocol errors in the read ExceptionGroup

The ModbusProtocolError branch logged but never appended to errs, so a
group where every attempt desynced raised
ValueError: second argument (exceptions) must be a non-empty sequence
instead of the Modbus failure.

Reachable today on a noisy RS485 link, and more so now that the identity
read shares this path.

Author: heinrich321

src/sunsynk/sunsynk.py

@@ -183,7 +183,8 @@ async def read_holding_registers(self, start: int, length: int) -> Sequence[int]
                     self.connection._params, ModbusSerialParams
                 ):
                     await self._flush_modbus_connection(reason=type(err).__name__)
-            except ModbusProtocolError:
+            except ModbusProtocolError as err:
+                errs.append(err)
                 _LOG.error(
                     "Read register %s x%s: protocol error [attempt %s/%s]",
                     start,

src/tests/sunsynk/test_sunsynk.py

@@ -6,6 +6,7 @@
 from unittest.mock import MagicMock, call, patch
 
 import pytest
+from modbus_connection import ModbusProtocolError
 
 from sunsynk import Sunsynk
 from sunsynk.rwsensors import NumberRWSensor
@@ -177,3 +178,14 @@ def rhr_side_effect(start: int, length: int) -> Sequence[int]:
     assert state[single] == 5
     assert state[pair] == 7 << 16
     assert state.registers == {1: 5, 10: 0, 11: 7}
+
+
+async def test_ss_protocol_error_raises_exception_group() -> None:
+    """Protocol errors must reach the ExceptionGroup, not leave it empty."""
+    unit = MagicMock()
+    unit.read_holding_registers.side_effect = ModbusProtocolError("desync")
+    ss = Sunsynk(unit=unit, read_attempts=2)  # type: ignore[arg-type]
+
+    with pytest.raises(ExceptionGroup) as excinfo:
+        await ss.read_holding_registers(622, 4)
+    assert len(excinfo.value.exceptions) == 2