Bug: Subsection prerequisite with min_score 0 is shown as gated by Studio but not enforced by the LMS

With enable_subsection_gating on, a subsection required as a prerequisite at minimum score 0, minimum completion 100 is reported as gated by Studio (xblock/outline gives visibility_state: "gated" and prereq_min_completion: 100), but the LMS does not gate it. A freshly enrolled learner's GET /api/courseware/sequence/<gated> returns gated_content.gated: false immediately.

min_score	min_completion	Studio visibility_state	LMS gated
0	100	gated	false
1	100	gated	true
50	0	gated	true

The gating milestone appears to be created only when min_score >= 1. A completion-only requirement produces no gate at all, and the author's completion threshold is silently ignored.

Expected: create the gating milestone whenever a completion requirement is set, or have Studio refuse or warn on a completion-only prerequisite it cannot enforce.