[widget_preview] Do not leak ListTileTheme into previewed widgets - #194992

In the widget preview scaffold, WidgetPreviewGroupWidget wrapped
ExpansionTile in a ListTileTheme. Because ListTileTheme is an
InheritedTheme (InheritedWidget), it enclosed ExpansionTile.children
(the previewed widgets). When previewed widgets build a ListTile or
SwitchListTile, ListTileTheme.of(context) resolved the ancestor
previewer's ListTileTheme because MaterialApp and Theme do not insert
a ListTileTheme boundary to reset ancestor inherited themes.

This caused previewed ListTiles and SwitchListTiles to render with an
unintended dark tileColor (#201E24), even when running inside a preview
with its own MaterialApp and ThemeData.

This change:
1. Removes the ListTileTheme wrapper around ExpansionTile in
   WidgetPreviewGroupWidget, passing dense: true directly to
   ExpansionTile and relying on expansionTileTheme for shape styling.
2. Removes the dead Theme(dividerColor: Colors.transparent) wrapper
   which also leaked into previews.
3. Removes the unused listTileTheme configuration from the scaffold's
   _baseTheme in theme.dart.
4. Adds automated integration regression tests in theming_test.dart.
5. Keeps template .tmpl files byte-identical to the scaffold source.

Previous output:
```
00:01 +4: Previewed MaterialApp ListTileTheme is not overridden
══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
The following TestFailure was thrown running a test:
Expected: MaterialColor:<MaterialColor(primary value: Color(alpha: 1.0000, red: 0.9569, green:
0.2627, blue: 0.2118, colorSpace: ColorSpace.sRGB))>
  Actual: Color:<Color(alpha: 1.0000, red: 0.1245, green: 0.1171, blue: 0.1394, colorSpace:
ColorSpace.sRGB)>
════════════════════════════════════════════════════════════════════════════════════════════════════
00:02 +4 -1: Previewed MaterialApp ListTileTheme is not overridden [E]

00:02 +4 -1: ListTileTheme does not leak into previewed widgets without MaterialApp
══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
The following TestFailure was thrown running a test:
Expected: null
  Actual: Color:<Color(alpha: 1.0000, red: 0.1245, green: 0.1171, blue: 0.1394, colorSpace:
ColorSpace.sRGB)>
════════════════════════════════════════════════════════════════════════════════════════════════════
00:02 +4 -2: ListTileTheme does not leak into previewed widgets without MaterialApp [E]

00:02 +4 -2: SwitchListTile in preview has transparent background by default
══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
The following TestFailure was thrown running a test:
Expected: has `color` with value Color:<Color(alpha: 0.0000, red: 0.0000, green: 0.0000, blue:
0.0000, colorSpace: ColorSpace.extendedSRGB)>
  Actual: ShapeDecoration:<ShapeDecoration(color: Color(alpha: 1.0000, red: 0.1245, green: 0.1171,
blue: 0.1394, colorSpace: ColorSpace.sRGB), shape: RoundedRectangleBorder(BorderSide(none),
BorderRadius.circular(12.0)))>
   Which: has `color` with value Color:<Color(alpha: 1.0000, red: 0.1245, green: 0.1171, blue:
0.1394, colorSpace: ColorSpace.sRGB)>
════════════════════════════════════════════════════════════════════════════════════════════════════
00:03 +4 -3: SwitchListTile in preview has transparent background by default [E]
```

Current output:
```
00:00 +0: loading dev/integration_tests/widget_preview_scaffold/test/theming_test.dart
00:00 +0: Theming is correctly propagated down to the previewed widget
00:00 +1: Default theme is used if no preview theme is specified
00:00 +2: BrightnessToggleButton correctly toggles preview brightness
00:01 +3: Updated brightness property doesn't override state set by BrightnessToggleButton
00:01 +4: Previewed MaterialApp ListTileTheme is not overridden
00:01 +5: ListTileTheme does not leak into previewed widgets without MaterialApp
00:01 +6: SwitchListTile in preview has transparent background by default
00:01 +7: All tests passed!
```

Fixes flutter#194660