Convert two bare db.rollback() calls on the shared per-request session
(best_layouts _fetch_supply_only_layouts + Step 2b core-attribution) to
db.begin_nested() — a bare rollback there discards ALL uncommitted work of
the request and orphans the outer SessionTransaction. Also wrap the swallow
sites in market_metrics (_query_stock, _query_sales_window, _price_sensitivity,
compute_offer_price_trend), best_layouts step-5 supply, future_supply
_query_supply_rows, own_portfolio _query_current/_query_future, and
custom_pois get_overlaps_for_scoring so a failed statement no longer poisons
the shared tx for later steps. Per-site regression tests assert fallback
preserved, begin_nested used, bare rollback NOT called, and session usable
after failure.
Refs #2464 (cluster A session-poisoning, Wave 2).
EOF2
echo "HANDLED"
