11 db.execute swallow sites across the _get_* helpers in
quarter_dump_lookup.py caught failures without a savepoint, aborting the
shared /analyze request session for sibling helpers and downstream
queries (get_or_fetch_zone_regulation, the 9f cad_parcels begin_nested
block). Wrap each in db.begin_nested(); _get_engineering_structures_by_
boundary and _get_zouit_engineering_overlaps get two independent
savepoints (one per db.execute) per the RELEASE SAVEPOINT trap.
get_connection_points and _get_parcel_wkt confirmed already-safe (their
db.execute propagates, not swallows). Adds regression tests.
Refs #2464 (cluster A session-poisoning, Wave 2).
