refactor(db-repository): extract folder query operations to FolderQueryRepository

[rafaeltonholo]

Contribution Summary

Linked Issue/Ticket: Resolves #11470

Description

• Add FolderQueryRepository interface with findById, findFolderServerIdById, findIdByServerId, and isPresent methods
• Add DefaultFolderQueryRepository implementation
• Remove folder query methods from FolderRepository interface
• Update consumers to use new repository: UnreadWidgetDataProvider, SettingsExporter, GetAccountFolders, MessageDetailsViewModel, K9BackendStorageFactory, and AutoExpandFolderBackendFoldersRefreshListener
• Make BackendFoldersRefreshListener.onAfterFolderListRefresh suspend function
• Make SettingsExporter.writeFolderNameSettings and UnreadWidgetRepository.upgradePreferences suspend functions
• Add comprehensive tests for DefaultFolderQueryRepository
• Remove FakeFolderRepository in favour of repository-specific fakes

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• This PR has a descriptive title and body that accurately outlines all changes made, and contains a reference to any issues that it fixes (e.g. Closes #XXX or Fixes #XXX).

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