Get Folder Contents

List a folder's direct children: subfolders, files, and forms filed directly in it (not further nested). Each item carries a `type` field (`folder`, `file`, or `form`) telling you what it is. For `type: "folder"` items, call this same endpoint again with that folder's id to walk further in. Forms are ordinary files under the hood (`isForm: true` on the underlying file) that inherit their folder/phase placement, so they appear here rather than in a separate forms listing. Note: on this endpoint (and the phase-contents endpoint), the file's MIME/category label is named `fileType` instead of `type`, to avoid colliding with the folder/file/form discriminator. This does not affect `GET /projects/{project_id}/files`, where `type` keeps its original meaning.