fix(antigravity): attach sibling tool images to the nearest functionResponse (#5075)

Responses→Gemini emits functionResponse plus sibling inline_data, but
fixCLIToolResponse previously dropped those images when regrouping tool
turns. Cloud Code Assist only sees tool images nested in
functionResponse.parts with an explicit mimeType, and parallel tool
results must stay bound to the nearest preceding functionResponse
instead of last-wins.

Closes #5070