ToolReturn.content reaches the model as a user-role message and the pictures
arrive bare, so nothing connects a figure to the chunk it came from:
BinaryContent.identifier does not survive serialization to the vision API, and
the captions in the result text correlate only by position.
Precede each picture with its position, source chunk id and self_ref.
build_binary_parts_from_results becomes build_image_content_from_results and
returns the labels interleaved with the pictures, so both attachment sites emit
them the same way.
This does not stop a model narrating retrieved pictures as user-supplied.
Measured on gemma4-26b with a single note ahead of the batch, and again with
per-image labels: it quotes the label and still says the user provided them.
The message role wins over its text.
