Enhance test coverage and error handling in test_helpers and test_rws…

…ensors

- Added error handling tests for `pack_value` in `test_helpers.py` to validate behavior for invalid inputs.
- Updated `test_rwsensors.py` to include a factor for signed values in `NumberRWSensor`, improving test accuracy for negative value handling.

src/tests/sunsynk/test_helpers.py

@@ -1,5 +1,7 @@
 """Test helpers."""
 
+import struct
+
 import pytest
 
 from sunsynk.helpers import (
@@ -119,6 +121,12 @@ def test_pack_unpack() -> None:
     assert unpack_value(pack_value(val, bits=16, signed=True), signed=True) == val
     assert unpack_value(pack_value(val, bits=32, signed=True), signed=True) == val
 
+    # Test error cases
+    with pytest.raises(struct.error):
+        pack_value(-1, bits=16, signed=False)
+    with pytest.raises(ValueError):
+        pack_value(1, bits=8)  # Invalid bit length
+
 
 def test_patch_bitmask() -> None:
     """Test patch_bitmask function."""

src/tests/sunsynk/test_rwsensors.py

@@ -94,6 +94,7 @@ def test_number_rw(state: InverterState) -> None:
 
     # writing negative values (when allowed by min)
     s.min = -10
+    s.factor = -1  # indicate signed values
     assert s.value_to_reg(-1, state.get) == (65535,)
 
     s = NumberRWSensor(1, "s2", factor=0.01)