Fix clear completed tasks being undone by immediate refresh

[saadkhalidkhan]

Summary

• Remove unnecessary refresh() after clearCompletedTasks() in TasksViewModel.
• The task list already updates via Room Flow.
• An immediate full refresh can reload stale network data before saveTasksToNetwork() finishes (the fake network has a 2s delay), which can restore completed tasks the user just cleared.

Test plan

• :app:testDebugUnitTest (TasksViewModelTest)
• Manual: complete tasks, Clear completed from menu, verify completed tasks stay cleared

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[gemini-code-assist]

Code Review

This pull request removes an unnecessary manual refresh call in the TasksViewModel after clearing completed tasks and updates the associated test to use advanceUntilIdle(). Feedback suggests improving the test's robustness by waiting for a specific state condition instead of using uiState.first(), which can be prone to race conditions when testing StateFlow.

[saadkhalidkhan]

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