[mike-engel]

This is another set of the App API backfill which adds the 10 Design Studio folder and email endpoints to APIClient (languages + components are in #240):

Method	Endpoint
listDesignStudioFolders	GET /v1/design_studio/folders
createDesignStudioFolder	POST /v1/design_studio/folders
getDesignStudioFolder	GET /v1/design_studio/folders/{id}
updateDesignStudioFolder	PUT /v1/design_studio/folders/{id} (204)
deleteDesignStudioFolder	DELETE /v1/design_studio/folders/{id} (204)
listDesignStudioEmails	GET /v1/design_studio/emails
createDesignStudioEmail	POST /v1/design_studio/emails
getDesignStudioEmail	GET /v1/design_studio/emails/{id}
updateDesignStudioEmail	PUT /v1/design_studio/emails/{id} (204)
deleteDesignStudioEmail	DELETE /v1/design_studio/emails/{id} (204)

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Note

Medium Risk
Large additive surface area and a shared HTTP change for multipart uploads; behavior is mostly thin wrappers over remote APIs with solid test coverage.

Overview
Extends App API coverage with Design Studio (folders, emails, email translations, components), Assets (files and folders), and matching docs/app.md reference.

Design Studio list calls use page-based filters/sorting (not cursor pagination), with typed inputs and the usual MissingParamError validation. Assets adds createAsset via multipart/form-data: the request layer gains postForm / formOptions (no caller Content-Type so fetch sets the boundary), and uploads derive MIME type from filename when needed so parts are not sent as application/octet-stream.

Unit tests cover query strings, validation, and form bodies; live integration tests exercise create/read/update/delete round-trips where the workspace allows.

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