fix: Improve the Google Drive / Sharepoint / OneDrive connector's validation and sync

[erichare]

This pull request improves the robustness and clarity of the Google Drive connector by adding better error handling, initialization checks, and more informative logging. The changes help prevent issues related to uninitialized services, stale shortcut data, and unclear authentication failures, making debugging and user experience smoother.

Error handling and initialization checks:

• Added checks to ensure self.service is initialized before performing operations like listing children, downloading files, resolving shortcuts, and listing files. Raises clear runtime errors if not initialized. [1] [2] [3] [4]
• In load_credentials, added handling for failed refresh token scenarios: clears credentials, removes invalid token file, and raises a clear error message prompting re-authentication.

Shortcut cache management:

• Introduced _clear_shortcut_cache() and ensured the shortcut cache is cleared at the start of _iter_selected_items to prevent stale shortcut resolution. [1] [2]

Improved logging and error surfacing:

• Enhanced logging to provide warnings when no files or folders are selected, or when no files are found after expansion/filtering, including possible reasons. [1] [2]
• Changed list_files to log errors with stack traces and re-raise exceptions, surfacing authentication and permission issues instead of silently swallowing them.