spp_api_v2: Location headers of POST /Individual, POST /Group and $split contain an unencoded '#' #563

Problem

Location is built as /api/v2/spp/Individual/{system}|{value} with the raw system URI
(urn:openspp:vocab:id-type#national_id). The # starts a URL fragment, so a client following the header requests
…/Individual/urn:openspp:vocab:id-type and gets 400/404.

• spp_api_v2/routers/individual.py (create, ~348)
• spp_api_v2/routers/group.py (create ~305, $split ~901)

PR #555 fixed the same defect for POST /ProgramMembership (_membership_location: URL-encode the system and value).

Proposal

URL-encode the identifier in these Location headers (shared helper), with tests that follow the returned Location.

Context

Found in the adversarial review of PR #555 (#554).