ensure_closed_code_fences previously only handled triple-backtick
(```) code-block fences. Single backtick (`) inline-code spans have
the same problem: an orphaned opening backtick causes the remainder
of the message to render as inline code on Discord and other platforms.
After balancing triple-backtick fences, strip complete ```...```
regions and count remaining standalone backtick markers. If odd,
append a closing backtick. Same trade-off as the triple-backtick fix:
a stray closing backtick may create a brief empty inline-code span,
which is far less harmful than the rest of the message being inline code.