Gemini thought summaries occasionally emit a bare ```thinking / ``````thinking
opener line as a between-summary delimiter. consumeGoogleStream appended
thought-part text verbatim to ThinkingContent, and structured thought parts
bypass the visible-channel leaked-reasoning healers, so the delimiter reached
both live display and persisted transcripts as fence spam.
Route thought-part text through a streaming ThinkingFenceStripper that drops
only a standalone reasoning-fence opener line (>=3 backticks + thinking/
reasoning). Language-tagged code fences, bare closers, and inline mentions are
preserved.
Fixes#8719
roboomp
d06e6a30b6