main() took MCP_TRANSPORT from the environment with a streamable-http
default, while the startup guard read settings.mcp_transport (default
stdio). With MCP_TRANSPORT unset and no MCP_API_KEY, the guard passed and
main() served HTTP unauthenticated. Production sets MCP_TRANSPORT (Dockerfile,
compose), so nothing changes there; unset now means stdio. CDI-1919.