refactor(canopy-medicaid): switch extract_person_ids to typed UUID when MemberContext.person_id becomes typed

extract_person_ids currently parses ctx.members[*].person_id as a string. If the orchestrator ever upgrades MemberContext.person_id to a typed PersonId newtype (per canopy_common::id), the helper should switch to the typed value and drop the string parse.

Conditional on the orchestrator-side typing refactor happening first.

Extracted from tma-subscriber-person-lookup.adoc § Potential Improvements.