security(spp_api_v2_programs): POST /ProgramMembership "Beneficiary not found: <ref>" 422 reveals registrant existence [Severity: Low] #560

Problem

ProgramMembershipService.from_api_schema() raises ValidationError("Beneficiary not found: <reference>")
(spp_api_v2_programs/services/program_membership_service.py, ~312), which the create/update routers return as 422.
A client holding program_membership:create but requiring consent can tell whether a registrant exists (422 "not
found" vs a created or otherwise-refused membership), which the read endpoints avoid by answering 403.

Proposal

For consent-requiring clients, answer an unknown beneficiary the same way as a beneficiary the client may not access
(403, jittered), and cover it with a test. Note that create-with-existing-identifier 409s (PR #555) are an accepted,
documented existence signal for create-scope clients; this issue is about the beneficiary reference on membership
create/update.

Context

Found in the adversarial review of PR #555 (#554); pre-existing.