extractCodeBlocks split on "```" and treated every odd-indexed part as a
closed code block. With an odd number of fences (>=3), Split returns an even
number of parts, so the final segment -- which follows an unpaired opening
fence and must stay literal -- landed on an odd index and was both emitted as
a \x00CB\x00 placeholder (later restored as ```seg```) AND re-appended
verbatim by the trailing len(parts)%2==0 block. The segment was therefore
duplicated in the rendered Slack message (and blocks gained a spurious entry).
Unclosed trailing fences are common in streamed LLM output.
Exclude the final unpaired part from the paired-scan loop. Add a regression
test covering the closed-block-then-unpaired-fence case.
Syed Osama Ali Shah