Check shared phone number availability

Checks whether a new shared iMessage number can be assigned to the given phone number under this project. Mirrors the allocation rules used by POST /projects/:projectId/users, including reuse of a soft-deleted user’s previously assigned number within the same project. Returns false only when the project has no soft-deleted slot to reuse and every Cosmos shared number is already assigned globally for this phone. Requires Authorization: Basic base64(projectId:projectSecret).