🧪 Add unit tests for NetworkService.getWifiUsageForPeriod

[LeanBitLab]

🎯 What: The missing tests for NetworkService.getWifiUsageForPeriod have been implemented.
📊 Coverage: The test coverage now includes the 'daily', 'weekly', 'monthly', and default/unknown period scenarios to guarantee correct delegation to the underlying FLauncherChannel.
✨ Result: Improved test coverage and reliability for network usage fetching.

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PR created automatically by Jules for task 5868886514666042722 started by @LeanBitLab

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