"""IMP-45 (#74) u3 — tests for :mod:`src.slide_css_injector`. Mirrors the inject_image_overrides_style test pattern in ``tests/test_image_id_stamper.py`` (lines 306-377) for the new ``slide_css`` axis. Eight cases : 1. empty css returns html unchanged 2. None css returns html unchanged (mirror of empty-css guard for the typed ``str | None`` signature) 3. inserts before (precedence path 1) 4. case-insensitive head close (precedence path 1, uppercase variant) 5. body-open fallback when no (precedence path 2) 6. document-start fallback when neither head nor body present (path 3) 7. idempotent on second call with identical css 8. replaces existing block when re-called with different css (latest-wins) Plus one additional guard : 9. marker comments wrap the injected ") < e