Adds `ensure_closed_code_fences()` helper to detect text with an odd
count of triple-backtick markers (indicating an unclosed code block)
and append a closing fence.
Applies the fix to all four identified gap paths:
G1: truncate_message early-break path for final chunk
G2: _send_or_edit streaming edit path (most commonly hit)
G3: overflow split first chunk (covered by G2's fix)
G4: _send_fallback_final fallback send path
Closes: #TBD
skywind
3e2fe3fcab