fix Room Exception on empty Note queries

[cka-dev]

fixes #40

[gemini-code-assist]

Summary of Changes

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This pull request addresses an issue where querying for a non-existent note could lead to an exception. It refactors the data access layer to return nullable Note objects from stream queries, providing a more robust and predictable way to handle cases where a note might not be found. This change propagates through the repository, DAO, and view models, enhancing the application's stability and user experience by preventing crashes related to empty query results and improving coroutine management within the HomeScreenViewModel.

Highlights

• Nullable Note Streams: The getNoteStream methods across NoteDao, NotesRepository, FakeNotesRepository, and OfflineNotesRepository have been updated to return Flow<Note?> instead of Flow<Note>. This change allows the data layer to gracefully handle cases where a requested note might not exist, preventing exceptions.
• ViewModel Robustness: The HomeScreenViewModel now properly handles the nullable Note from the data layer, using safe calls (?.let) and updating UI state accordingly. It also introduces a selectNoteJob to manage and cancel ongoing note selection coroutines, preventing race conditions and improving responsiveness.
• Exception Handling: Specific handling for CancellationException has been added in HomeScreenViewModel to ensure that coroutine cancellations are re-thrown and not caught as general exceptions, maintaining proper coroutine lifecycle management.
• UI State Management: When a note is deleted or its favorite status is toggled, if it's the currently selected note, the HomeScreenViewModel now explicitly updates the UI state to reflect the change or clear the selection, ensuring UI consistency.
• Test Updates: All relevant test files (NoteDaoTest, CanvasScreenViewModelTest, HomeScreenViewModelTest) have been updated to accommodate the nullable Note return type, using safe calls (?.title, ?.isFavorite) or non-null assertion operators (!!) where appropriate.

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[gemini-code-assist]

Code Review

This pull request successfully addresses a potential crash from Room by making note queries nullable. The changes are consistently applied across the data layer, ViewModels, and tests. The introduction of coroutine job management in HomeScreenViewModel is a great improvement for lifecycle handling and robustness. I've found one minor formatting issue in a test file that should be addressed.