Testing strategy: unit, integration, WebSocket; add JaCoCo coverage targets

[sugan0tech]

• Description: Establish testing layers and coverage gates.
• Current state: Only GatherspaceApplicationTests exists; no JaCoCo; no WebSocket tests.
• Tasks:
  • Add JaCoCo plugin with a minimum coverage threshold (e.g., 70%).
  • Unit tests for services (e.g., SpaceService, UserSessionServiceImpl, JwtUtil).
  • MVC integration tests for controllers; WebSocket tests using Spring’s WebSocketClient against /ws.
  • Document test strategy in README.
• Acceptance criteria:
  • mvn test enforces coverage and passes locally and in CI.
• References: pom.xml, services/controllers under features/*.