search_documents and search_documents_by_image return a ToolResult: the
format_for_agent rendering with rank, Document ID and Collection so the
text alone drives the document tools; one ImageContent per distinct
picture, labelled with its result; and the SearchResult list without
image_data as structured content. format_for_agent gains an opt-in
include_document_id, so the capabilities' rendering is unchanged.
collect_pictures is the one place pictures are deduplicated and
validated for both wire formats.
Refs #599
5653e876a8