BDC borrower name autocomplete

GET /api/bdc/autocomplete response. F4 schema-tightening (2026-04-30): results REQUIRED. Every code path (empty-query, success, exception fallback) returns {"results": [...]} — even the catch-all returns {"results": []} (no status_code=500 declared, so it's a 200). No required-fields gate on the items (autocomplete consumers are forgiving — show any matches the upstream produces).

Frontend BDC borrower-search overlay iterates data.results.forEach(...) to render suggestion rows.