Fix sensors getting wrong unit from MeasuredValueType attribute in Overkiz

[iMicknl]

Proposed change

Overkiz provides a device-level core:MeasuredValueType attribute which links to the 'main sensor', we don't know what the main sensor is and we should check if the value type is valid for this device class.

Fixes #140651

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 #
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[Copilot]

Pull request overview

This PR fixes incorrect sensor units in the Overkiz integration when a device-level core:MeasuredValueType attribute (describing a device’s “main” measurement) was incorrectly applied to unrelated sensors (e.g., RSSI), causing invalid units for the sensor’s device class.

Changes:

• Validate core:MeasuredValueType-derived units against the sensor’s device_class (via homeassistant.components.sensor.DEVICE_CLASS_UNITS) before using them.
• Keep per-state <stateName>MeasuredValueType behavior unchanged (still takes precedence).
• Update Overkiz sensor snapshot expectations for RSSI sensors to use the correct signal strength unit.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
homeassistant/components/overkiz/sensor.py	Only applies device-level core:MeasuredValueType unit overrides when the resolved unit is valid for the sensor’s device class.
tests/components/overkiz/snapshots/test_sensor.ambr	Updates snapshots to reflect corrected RSSI unit of measurement (dB).