`_attach_picture_data` fetched picture bytes once per document, over the
`limit * 10` candidates reranking asks for, so it was the per-document fetch with
the most candidates behind it. It now issues one query however many documents the
candidates span: one for ten documents, as for one.
Removes `get_text_for_refs`, whose only caller now gets the text back with the
bytes from `get_pictures_grouped`.
`test_client_search_include_images_false_skips_lookup` returned no search
results, so asserting the picture accessor went uncalled held whatever the code
did. It now returns a picture-carrying result, making "did not fetch" the
assertion rather than "had nothing to fetch".
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