🧪 [testing improvement] Add tests for SettingsService setDateTimeFormat

[LeanBitLab]

🎯 What: The testing gap addressed was that SettingsService.setDateTimeFormat lacked unit tests in test/providers/settings_service_test.dart.

📊 Coverage: Tests were added for two main scenarios:

1. Verifying that the method correctly stores and exposes the dateFormat and timeFormat arguments.
2. Verifying that notifyListeners is triggered exactly once per execution to ensure UI rebuilding.

✨ Result: Test coverage improved by properly validating that the method correctly performs dual configuration persistence and event notification.

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PR created automatically by Jules for task 9810824321961271679 started by @LeanBitLab

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