Center Floating Snackbar with custom width when direction is RTL (#140215)

## Description

This PR fixes the positionning of a floating snackbar when the text direction is RTL.

## Related Issue

Fixes https://github.com/flutter/flutter/issues/140125.

## Tests

Adds 1 test.

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

@@ -1183,7 +1183,13 @@ class _ScaffoldLayout extends MultiChildLayoutDelegate {
           : contentBottom;
       }
 
-      final double xOffset = hasCustomWidth ? (size.width - snackBarWidth!) / 2 : 0.0;
+      double xOffset = 0.0;
+      if (hasCustomWidth) {
+        xOffset = switch (textDirection) {
+          TextDirection.rtl => (snackBarWidth! - size.width) / 2,
+          TextDirection.ltr => (size.width - snackBarWidth!) / 2,
+        };
+      }
       positionChild(_ScaffoldSlot.snackBar, Offset(xOffset, snackBarYOffsetBase - snackBarSize.height));
 
       assert((){

packages/flutter/test/material/snack_bar_test.dart

@@ -2604,6 +2604,49 @@ void main() {
         expect(snackBarTopRight.dx - actionTopRight.dx, 8.0 + 15.0); // button margin + horizontal scaffold outside margin
       },
     );
+
+    testWidgets('Floating snackbar with custom width is centered when text direction is rtl', (WidgetTester tester) async {
+      // Regression test for https://github.com/flutter/flutter/issues/140125.
+      const double customWidth = 400.0;
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Directionality(
+            textDirection: TextDirection.rtl,
+            child: Scaffold(
+              body: Builder(
+                builder: (BuildContext context) {
+                  return GestureDetector(
+                    onTap: () {
+                      ScaffoldMessenger.of(context).showSnackBar(
+                        const SnackBar(
+                          behavior: SnackBarBehavior.floating,
+                          width: customWidth,
+                          content: Text('Feeling super snackish'),
+                        ),
+                      );
+                    },
+                    child: const Text('X'),
+                  );
+                },
+              ),
+            ),
+          ),
+        ),
+      );
+
+      await tester.tap(find.text('X'));
+      await tester.pump(); // start animation
+      await tester.pump(const Duration(milliseconds: 750));
+
+      final Finder materialFinder = find.descendant(
+        of: find.byType(SnackBar),
+        matching: find.byType(Material),
+      );
+      final Offset snackBarBottomLeft = tester.getBottomLeft(materialFinder);
+      final Offset snackBarBottomRight = tester.getBottomRight(materialFinder);
+      expect(snackBarBottomLeft.dx, (800 - customWidth) / 2); // Device width is 800.
+      expect(snackBarBottomRight.dx, (800 + customWidth) / 2); // Device width is 800.
+    });
   });
 
   testWidgets('SnackBars hero across transitions when using ScaffoldMessenger', (WidgetTester tester) async {