`SelectableText.rich`: Fix `TextSpan.style` not merged into default text style (#101911)

Properly merges the TextSpan.style of SelectableText.rich.

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

@@ -664,7 +664,7 @@ class _SelectableTextState extends State<SelectableText> implements TextSelectio
     final DefaultTextStyle defaultTextStyle = DefaultTextStyle.of(context);
     TextStyle? effectiveTextStyle = widget.style;
     if (effectiveTextStyle == null || effectiveTextStyle.inherit) {
-      effectiveTextStyle = defaultTextStyle.style.merge(widget.style);
+      effectiveTextStyle = defaultTextStyle.style.merge(widget.style ?? _controller._textSpan.style);
     }
     if (MediaQuery.boldTextOverride(context)) {
       effectiveTextStyle = effectiveTextStyle.merge(const TextStyle(fontWeight: FontWeight.bold));

packages/flutter/test/widgets/selectable_text_test.dart

@@ -5052,4 +5052,24 @@ void main() {
     expect(find.text('Regular Text', skipOffstage: false), findsOneWidget);
     expect(find.byType(SelectableText, skipOffstage: false), findsOneWidget);
   });
+
+  testWidgets('SelectableText text span style is merged with default text style', (WidgetTester tester) async {
+    // This is a regression test for https://github.com/flutter/flutter/issues/71389
+
+    const TextStyle textStyle = TextStyle(color: Color(0xff00ff00), fontSize: 12.0);
+
+    await tester.pumpWidget(
+      const MaterialApp(
+        home: SelectableText.rich(
+          TextSpan(
+            text: 'Abcd',
+            style: textStyle,
+          ),
+        ),
+      ),
+    );
+
+    final EditableText editableText = tester.widget(find.byType(EditableText));
+    expect(editableText.style.fontSize, textStyle.fontSize);
+  });
 }