Returns address completion previews for a partial address string, suitable
for populating an autocomplete dropdown.

Regular mode — supply partialStreet to search by partial street address.
Results may include Address types (resolvable directly) and Container types
(buildings/complexes that require a follow-up call).

Advanced mode — supply advanced=true and a container ID (from a previous
regular call) to drill into a building or complex and retrieve individual unit addresses.

Results with type: "Address" can be fully resolved by passing their id
to POST /completions.

• Does not consume a lookup.