Make UnitSystem a frozen dataclass

[thecode]

Proposed change

CI was failing since tests were modifying the UnitSystem metric constant which affected all tests if it is not set back to the original values after the test is finished, discussion about it suggested to make it a frozen data class.

Background: #140295, #140323

CI tests fixed already at: #140376, #140372, #140371, #140366, #140365, #140358, #140357 which can be seen that CI pass here for all tests.

With a frozen data class if a test modify the UnitSystem constant it fails (example of reverting #140365 locally):

E     dataclasses.FrozenInstanceError: cannot assign to field 'temperature_unit'

<string>:24: FrozenInstanceError

tests/components/lg_thinq/test_climate.py ⨯                                                                                                                         
FAILED tests/components/lg_thinq/test_climate.py::test_all_entities - dataclasses.FrozenInstanceError: cannot assign to field 'temperature_unit'

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:
• Link to developer documentation pull request: Add blog post about changes to UnitSystem class and their implications developers.home-assistant#2618
• Link to frontend pull request:

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• I have followed the perfect PR recommendations
• The code has been formatted using Ruff (ruff format homeassistant tests)
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

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• For the updated dependencies - a link to the changelog, or at minimum a diff between library versions is added to the PR description.

To help with the load of incoming pull requests:

• I have reviewed two other open pull requests in this repository.

[MartinHjelmare]

Do we need to search for published custom integrations that would be affected by this change? We've seen before that custom integrations do weird things.

[thecode]

Do we need to search for published custom integrations that would be affected by this change? We've seen before that custom integrations do weird things.

I have tried to think of it, I can't be 100% sure, but I think if a custom integration modify the unit system it will already have undesired effects since this is the base for unit conversions and it globally used by all sensors so I assume this was only a bad pattern we used in tests.

[MartinHjelmare]

Yes, it would already be bad, but it could be a hidden bug, while now it will raise an error immediately if the integration tries to modify the instance.

We should probably at least publish a dev blog about the change.

[thecode]

We should probably at least publish a dev blog about the change.

Yes, we should

[MartinHjelmare]

Thanks!