fix: `ExpansionTileTheme.shape` assignment in `ExpansionTile` (#127749)

The ExpansionTile was not following `shape` from ExpansionTileTheme as it was assigning wrong parameter to the tween.

fixes #129785

packages/flutter/lib/src/material/expansion_tile.dart

@@ -683,7 +683,7 @@ class _ExpansionTileState extends State<ExpansionTile> with SingleTickerProvider
           bottom: BorderSide(color: Colors.transparent),
         )
       ..end = widget.shape
-        ?? expansionTileTheme.collapsedShape
+        ?? expansionTileTheme.shape
         ?? Border(
           top: BorderSide(color: theme.dividerColor),
           bottom: BorderSide(color: theme.dividerColor),

packages/flutter/test/material/expansion_tile_theme_test.dart

@@ -290,7 +290,7 @@ void main() {
     // Check the expanded text color when textColor is applied.
     expect(getTextColor(), textColor);
     // Check the expanded ShapeBorder when shape is applied.
-    expect(shapeDecoration.shape, collapsedShape);
+    expect(shapeDecoration.shape, shape);
 
     // Check the child position when expandedAlignment is applied.
     final Rect childRect = tester.getRect(find.text('Tile 1'));