Add sensor platform to fressnapf_tracker

[eifinger]

Proposed change

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: Add sensor platform to fressnapf_tracker home-assistant.io#42048
• 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.
• The code change is tested and works locally.
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• The code has been formatted using Ruff (ruff format homeassistant tests)
• Tests have been added to verify that the new code works.
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  Updated and included derived files by running: python3 -m script.hassfest.
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  Updated by running python3 -m script.gen_requirements_all.
• For the updated dependencies - a link to the changelog, or at minimum a diff between library versions is added to the PR description.

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[Copilot]

Pull request overview

This PR adds a battery sensor platform to the Fressnapf Tracker integration, complementing the existing device tracker functionality. The sensor monitors the battery percentage of GPS pet trackers.

Key Changes

• Added a new sensor platform for battery level monitoring
• Created a reusable entity base class to reduce code duplication
• Refactored test infrastructure to support multiple platforms with proper platform isolation

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
homeassistant/components/fressnapf_tracker/__init__.py	Added Platform.SENSOR to the PLATFORMS list to enable sensor setup
homeassistant/components/fressnapf_tracker/sensor.py	Implemented the sensor platform with battery entity support using coordinator pattern
homeassistant/components/fressnapf_tracker/entity.py	Added FressnapfTrackerEntity base class to handle entity descriptions and unique ID generation for sensor entities
homeassistant/components/fressnapf_tracker/strings.json	Added translation configuration for the battery sensor using standard battery component key
tests/components/fressnapf_tracker/test_sensor.py	Created comprehensive snapshot tests for the new sensor platform
tests/components/fressnapf_tracker/test_device_tracker.py	Refactored to isolate device tracker platform testing and extract common device snapshot logic
tests/components/fressnapf_tracker/__init__.py	Added reusable snapshot_devices helper function to reduce test code duplication
tests/components/fressnapf_tracker/snapshots/test_sensor.ambr	Added snapshot data for entity and device registry verification

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