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>