Fix getOffsetForCaret crash (#146669)

Fixes a crash in Google tests (b/333560406) related to a decomposed complex character.

Author: LongCatIsLooong

packages/flutter/lib/src/painting/text_painter.dart

@@ -1468,10 +1468,15 @@ class TextPainter {
     final _LineCaretMetrics metrics;
     final List<TextBox> boxes = cachedLayout.paragraph
       .getBoxesForRange(graphemeRange.start, graphemeRange.end, boxHeightStyle: ui.BoxHeightStyle.strut);
+
     if (boxes.isNotEmpty) {
-      final TextBox box = boxes.single;
+      final bool ahchorToLeft = switch (glyphInfo.writingDirection) {
+        TextDirection.ltr => anchorToLeadingEdge,
+        TextDirection.rtl => !anchorToLeadingEdge,
+      };
+      final TextBox box = ahchorToLeft ? boxes.first : boxes.last;
       metrics = _LineCaretMetrics(
-        offset: Offset(anchorToLeadingEdge ? box.start : box.end, box.top),
+        offset: Offset(ahchorToLeft ? box.left : box.right, box.top),
         writingDirection: box.direction,
       );
     } else {

packages/flutter/test/painting/text_painter_test.dart

@@ -1682,6 +1682,20 @@ void main() {
     });
   }, skip: kIsWeb && !isSkiaWeb); // [intended] strut spport for HTML renderer https://github.com/flutter/flutter/issues/32243.
 
+  test('getOffsetForCaret does not crash on decomposed characters', () {
+    final TextPainter painter = TextPainter(
+      textDirection: TextDirection.ltr,
+      text: const TextSpan(
+        text: '각',
+        style: TextStyle(fontSize: 10),
+      ),
+    )..layout(maxWidth: 1); // Force the jamo characters to soft wrap.
+    expect(
+      () => painter.getOffsetForCaret(const TextPosition(offset: 0), Rect.zero),
+      returnsNormally,
+    );
+  });
+
   test('TextPainter dispatches memory events', () async {
     await expectLater(
       await memoryEvents(() => TextPainter().dispose(), TextPainter),