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.

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.

Start by tracing the xblock/outline visibility_state and the LMS GET /api/courseware/sequence/ response for subsection prerequisites. Compare the min_score 0, min_completion 100 case with the working cases in the issue, then add regression coverage for completion-only gating. Done means the LMS enforces the completion threshold consistently with Studio, or the unsupported configuration is refused or warned about.