Publishes a blog post to dev.to.

Args:
    title (str): The title of the blog post.
    body_markdown (str): The content of the blog post in Markdown format.
    tags (Optional[List[str]]): A list of tags for the blog post (e.g., ["python", "webdev"]).
    published (bool): Set to True to publish immediately, False to save as a draft.
    series (Optional[str]): The name of the series this article belongs to.
    canonical_url (Optional[str]): The canonical URL of the article if it's cross-posted.
    cover_image (Optional[str]): URL of the cover image for the article.

Returns:
    str: A message indicating the success or failure of the publishing operation,
         including the article URL if successful.