test: replace the disabled convex_hull_2d test (#14892)

test(libslic3r): replace the disabled convex_hull_2d test, closing #11269

The last "failing libslic3r test" from #11269 was the disabled
SCENARIO("2D convex hull of sinking object", "[3mf][.]") in test_3mf.cpp.
It checked ModelObject::convex_hull_2d for a sinking object against
PrusaSlicer's reference hull, but Orca's convex_hull_2d does not clip
geometry below the bed the way PrusaSlicer's its_convex_hull_2d_above does,
so the reference never matched. The test also wrote a debug mesh to a
hardcoded /tmp path and its comparison loop was inverted.

Remove it and add tests/libslic3r/test_model.cpp characterizing
convex_hull_2d on non-sinking transforms (identity and scale+offset),
where the projected footprint is unambiguous. Homed in a Model test file
since it exercises ModelObject, not 3MF.

tests/libslic3r/test_model.cpp

@@ -0,0 +1,40 @@
+#include <catch2/catch_all.hpp>
+
+#include "libslic3r/Model.hpp"
+
+using namespace Slic3r;
+
+// convex_hull_2d does not clip geometry below the bed, so these cases avoid
+// sinking transforms.
+TEST_CASE("A part's 2D convex hull is its footprint projected onto the bed", "[Model]")
+{
+    Model model;
+    ModelObject* object = model.add_object();
+    // Keep the cube's raw coordinates ([0,20] on every axis): the default
+    // add_volume re-centers the geometry, which would move the footprint.
+    object->add_volume(make_cube(20, 20, 20), ModelVolumeType::MODEL_PART, false);
+
+    SECTION("identity transform yields the 20 mm square") {
+        const Polygon hull   = object->convex_hull_2d(Geometry::Transformation{}.get_matrix());
+        const BoundingBox bb = hull.bounding_box();
+        CHECK(hull.size() == 4);
+        CHECK(bb.min.x() == scaled(0.));
+        CHECK(bb.min.y() == scaled(0.));
+        CHECK(bb.max.x() == scaled(20.));
+        CHECK(bb.max.y() == scaled(20.));
+    }
+
+    SECTION("scaling and offset move and grow the footprint") {
+        Geometry::Transformation t;
+        t.set_scaling_factor({2, 2, 2}); // cube now spans [0,40]
+        t.set_offset({10, 5, 0});        // then shift +10 in X, +5 in Y
+
+        const Polygon hull   = object->convex_hull_2d(t.get_matrix());
+        const BoundingBox bb = hull.bounding_box();
+        CHECK(hull.size() == 4);
+        CHECK(bb.min.x() == scaled(10.));
+        CHECK(bb.min.y() == scaled(5.));
+        CHECK(bb.max.x() == scaled(50.));
+        CHECK(bb.max.y() == scaled(45.));
+    }
+}