🧪 Fix WallpaperService tests and NoFileExplorerException coverage

[LeanBitLab]

🎯 What: The WallpaperService tests were failing due to missing stubs for timeBasedWallpaperEnabled and outdated gradient expectations. While the test for NoFileExplorerException existed, it was failing due to these issues.

📊 Coverage: Fixed the initialization mock for MockSettingsService so the NoFileExplorerException test and gradient tests can pass successfully.

✨ Result: Improved reliability and coverage of the codebase by getting the tests for WallpaperService to pass.

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PR created automatically by Jules for task 12789654614130269587 started by @LeanBitLab

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