fix Overlay sizing in widget_harness for Flutter 3.27+

Flutter 3.27 changed Overlay layout: an Overlay given infinite
height constraints now requires at least one OverlayEntry with
`canSizeOverlay: true` to delegate sizing, otherwise the entire
golden suite throws "Overlay was given infinite constraints" before
any test can render.

Marking the harness's only entry as size-determining is the minimal
fix — keeps the existing MediaQuery-driven layout shape intact and
unblocks every widget/golden test that uses `harness()`.

Co-Authored-By: Claude <noreply@anthropic.com>

.pql/pql-plan.json

@@ -1,5 +1,5 @@
 {
-  "exported_at": "2026-05-06T20:28:58Z",
+  "exported_at": "2026-05-06T20:29:05Z",
   "decisions": [
     {
       "id": "D-1",

test/helpers/widget_harness.dart

@@ -22,7 +22,10 @@ Widget harness(KernelFixture fixture, Widget child) {
           data: const MediaQueryData(),
           child: Overlay(
             initialEntries: [
-              OverlayEntry(builder: (_) => child),
+              OverlayEntry(
+                canSizeOverlay: true,
+                builder: (_) => child,
+              ),
             ],
           ),
         ),