- extract_images: only remove extracted image tags from content, preserve
non-image markdown links (e.g. PDFs) that were previously silently lost
- truncate_message: walk only chunk_body (not prepended prefix) so the
reopened code fence does not toggle in_code off, leaving continuation
chunks with unclosed code blocks
- Add 49 unit tests covering MessageEvent command parsing, extract_images,
extract_media, truncate_message code block handling, and _get_human_delay
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