Fix flaky modbus tests

[cdce8p]

Proposed change

Addresses flaky modbus test behavior on Python 3.13 discovered in #129442.

>       assert caplog.text == ""
E       AssertionError: assert '2024-11-09 2... of 2 bytes\n' == ''
E         
E         + 2024-11-09 21:32:13.801 ERROR    MainThread homeassistant.components.modbus.entity:entity.py:241
    Received 4 bytes, unpack error unpack requires a buffer of 2 bytes
tests/components/modbus/test_init.py:136: AssertionError

Background
Python 3.13 changed the event loop scheduler slightly. Under specific circumstances, this caused the async_update method to be run which wouldn't be execute in time for 3.12 to hit the caplog.text == "" assert. Thus the error that also exists on 3.12 just wouldn't be shown.

The error was emitted here:

core/homeassistant/components/modbus/entity.py

Lines 230 to 240 in 0fc0193

	byte_string = b"".join([x.to_bytes(2, byteorder="big") for x in registers])
	if self._data_type == DataType.STRING:
	return byte_string.decode()
	if byte_string == b"nan\x00":
	return None
	try:
	val = struct.unpack(self._structure, byte_string)
	except struct.error as err:
	recv_size = len(registers) * 2
	msg = f"Received {recv_size} bytes, unpack error {err}"

The register values are converted to a 2 bytes long byte and according to the error message a byte buffer of size 2 is expected. Therefore the register_words fixture should probably only contain one byte to begin with.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request:

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[cdce8p]

The tests for 3.13 pass as well (in the 3.13 PR): https://github.com/home-assistant/core/actions/runs/11759616361/job/32759371921?pr=129442#step:10:48