Fix BIA fallback hit extraction (avoid Untitled results)

[hugokallander]

Summary

• Fix fallback parsing for BioImage Archive nested hit schema (hits.hits[*]._source).
• Extract dataset title/accession from _source.title and _source.accession_id.
• Extract richer image metadata (file pattern, dataset UUID, accession, score).
• Return richer structured result objects so the agent can summarize concrete entries instead of Untitled.

Why

Endpoint responses are nested and were previously read using top-level keys (title, accession), which produced empty fields and Untitled outputs.

Validation

• pnpm -s tsc --noEmit