Update Google Health sensor units to be more reasonable

[allenporter]

Proposed change

Add dynamic unit suggestion logic for Google Health sensors via a new suggested_unit_fn callback on GoogleHealthSensorEntityDescription.

By default, Home Assistant does not convert weights/masses across unit systems automatically (to prevent body scales from defaulting to grams), and generic unit system mappings for distance and volume convert to architectural/utility defaults (m -> ft, L -> gal).

This PR configures unit-system-aware defaults tailored for a fitness/health tracker:

• Weight sensor: Defaults to pounds (lb) for US Customary users and kilograms (kg) for Metric users.
• Distance sensor: Defaults to miles (mi) for US Customary users and kilometers (km) for Metric users (instead of feet or raw meters).
• Hydration / Water intake sensor: Defaults to fluid ounces (fl. oz.) for US Customary users (instead of gallons).
• Parameterized unit testing: Added test_sensor_unit_conversions with pytest.approx() to test unit system conversions across METRIC_SYSTEM and US_CUSTOMARY_SYSTEM.

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:
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• Link to developer documentation pull request:
• Link to frontend pull request:

Checklist

• I understand the code I am submitting and can explain how it works.
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• Documentation added/updated for www.home-assistant.io

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