Previously, `StorageService.saveConnection` allowed multiple connections to be marked as `defaultForUploads: true` because it did not unset the flag on existing connections. This caused `loadFromStorage` (which uses `find`) to potentially pick up an old, invalid connection instead of the newly configured one, leading to a loop where the user is asked to configure credentials again.
This change modifies `saveConnection` to iterate through existing connections and unset `defaultForUploads` if the new connection is being saved as the default. This ensures the "default" invariant is maintained.
Added a unit test case to verify this behavior.
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