Backfill DEFAULT_THINKING_AG_SIGNATURE onto functionCall parts missing it
(client history replay) and on Claude tool_use blocks, fixing 400
INVALID_ARGUMENT from Gemini-family APIs. Suppress the OpenAI-style
data: [DONE] sentinel for antigravity/gemini/vertex to avoid parser crashes.
Fixes#2193.
Co-authored-by: Cursor <cursoragent@cursor.com>