Widen #48476's fence guarantees to the two splitters that still emitted
fence-broken chunks:
* GatewayStreamConsumer._split_text_chunks (fallback final send): close
the orphaned ``` at each chunk boundary and reopen it — with the
original language tag — on the next chunk, mirroring
BasePlatformAdapter.truncate_message's contract. Headroom is reserved
so balanced chunks stay within the platform limit.
* Slack block_kit._split_text (3000-char section chunking): same
close/reopen balancing for mrkdwn section text carrying fences.
With these, every chunk boundary — non-streaming send
(truncate_message), streaming overflow (_truncate_for_stream via
adapter.truncate_message per #45938), fallback final
(_split_text_chunks), final-send balance (ensure_closed_code_fences),
and Block Kit section splits — delivers fence-balanced chunks.
Regression tests probe each path with fenced fixtures, assert per-chunk
balance, limit compliance, language-tag reopening, and prose passthrough.
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.
