Fix #161953: Avoid copying edit mesh wrapper for shape keys

Entering Edit Mode on a mesh with shape keys could copy the final
mesh component after it had already been shared with the edit cage.

The generic component copy did not preserve all
runtime state required by the Edit Mode BMesh wrapper.
The copied final result was subsequently handled by
`MeshRenderData` through the regular `MeshExtractType::Mesh` path,
as though it were an array-backed mesh.
Viewport extraction then attempted to access mesh arrays that
were not available and asserted in `Mesh::faces()`.

Assign the evaluated shape key before sharing the final mesh
component with either the explicit or fallback cage. In Object Mode,
retrieve the final mesh pointer after potential writable
access so copy-on-write cannot leave a stale pointer.

Also add an undo test that would have caught this regression.

Ref !161986

tests/python/ui_simulate/test_undo.py

@@ -258,6 +258,49 @@ def _view3d_startup_area_single(e):
     assert len(e.window.screen.areas) == 1
 
 
+def view3d_mesh_edit_shape_key():
+    """Test drawing the final shape-key BMesh wrapper in Edit Mode and basic undo/redo."""
+    e, t, window = ui.test_window()
+    yield e.shift.f5()                  # 3D Viewport.
+    yield e.ctrl.alt.space()            # Full-screen.
+
+    def assert_state(mode, key_count, x_bounds):
+        cube = window.view_layer.objects.active
+        t.assertEqual(cube.name, "Cube")
+        t.assertEqual(cube.mode, mode)
+        shape_keys = cube.data.shape_keys
+        t.assertEqual(0 if shape_keys is None else len(shape_keys.key_blocks), key_count)
+        t.assertEqual(
+            (min(co[0] for co in cube.bound_box), max(co[0] for co in cube.bound_box)),
+            x_bounds,
+        )
+
+    assert_state('OBJECT', 0, (-1.0, 1.0))
+
+    yield from ui.call_operator(e, "Add Shape Key")  # Basis.
+    yield from ui.call_operator(e, "Add Shape Key")  # Key 1.
+
+    yield e.tab()                       # Edit mode.
+    assert_state('EDIT', 2, (-1.0, 1.0))
+    yield                               # Draw the evaluated BMesh wrapper.
+    yield e.a()                         # Select all.
+    yield e.g().x().two().ret()         # Translate the active shape key.
+    yield e.tab()                       # Object mode.
+    assert_state('OBJECT', 2, (1.0, 3.0))
+    yield from ui.call_operator(e, "Apply to Basis Key")
+
+    yield e.tab()                       # Edit mode.
+    yield                               # Draw the final evaluated BMesh wrapper.
+    assert_state('EDIT', 1, (1.0, 3.0))
+
+    yield e.ctrl.z(8)                   # Undo to the initial state.
+    assert_state('OBJECT', 0, (-1.0, 1.0))
+
+    yield e.ctrl.shift.z(8)             # Redo to the final state.
+    yield                               # Redraw after restoring Edit Mode.
+    assert_state('EDIT', 1, (1.0, 3.0))
+
+
 def view3d_simple():
     e, t, window = ui.test_window()
     yield from _view3d_startup_area_maximized(e)