Allow null LED bar mode in config responses

[lin-hongkuan]

Summary

Some AirGradient devices can return "ledBarMode": null from /config while the rest of the configuration payload is valid. The current model requires led_bar_mode to always be a LedBarMode, so deserialization raises InvalidFieldValue before consumers can use the rest of the response.

Allow Config.led_bar_mode to be None and add a regression test for that payload shape. This keeps existing enum values unchanged and still requires callers to pass a real LedBarMode when setting the LED bar mode.

This is the dependency-layer fix for home-assistant/core#168434.

Validation

• .venv\\Scripts\\python.exe -m pytest -o addopts=--cov=airgradient tests -q
• .venv\\Scripts\\ruff check src tests
• .venv\\Scripts\\ruff format --check src tests
• .venv\\Scripts\\mypy src

[lin-hongkuan]

The failing Verify check appears to be the PR label gate from .github/workflows/pr-labels.yaml. This should be labeled �ugfix, but I don't have permission to add labels to the upstream repository. Could a maintainer please add the �ugfix label?

[lin-hongkuan]

The Verify job is failing only because this PR does not currently have one of the required type labels. I do not have permission to add labels in this repository; \�ugfix\ seems appropriate for this change.