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Builds the Rich Markdown style source string for this list of RichBlocks. The resulting string may be passed to InputRichMessageMarkdown.
Media blocks (RichBlockPhoto, RichBlockVideo, RichBlockAudio, RichBlockVoiceNote and RichBlockAnimation) are rendered using the Telegram file_id as the media source. Telegram only accepts HTTP(S) URLs for rich media, so for those blocks the output is a faithful structural representation rather than a directly sendable message.