Add regression tests for onboarding.party wizard-completion checks

Add automated test coverage for the onboarding.party wizard-completion mechanism introduced in PR #1491:

• auth_handler.hpp's single-party login path and account_handler.hpp's select_party path: cases for onboarding.party=false (party_setup_required=true), onboarding.party=true + party Active (both flags false/empty), and onboarding.party=true + party still Inactive (party_setup_required=false, party_setup_warning set).
• system_setting_handler::complete_party_onboarding — the new permission-check-free NATS handler.
• system_settings_service's new is_onboarding_*_complete=/=set_onboarding_*_complete accessors.

None of this had test coverage before or after PR #1491 (matching the existing untested bootstrap_mode precedent, but flagged during review as worth closing given this is the second time a party-provisioner-wizard re-trigger bug has been fixed in this area). A regression test would lock in the fix and catch future drift in the party.status vs onboarding.party distinction.

• PR #1491: https://github.com/OreStudio/OreStudio/pull/1491

• Replace party.status Active/Inactive check with a party-level variability flag