Fix text range logic for a11y (#16496)

Make sure that a text range at the end of the string is still valid.

fml/platform/darwin/string_range_sanitization.mm

@@ -7,7 +7,7 @@
 namespace fml {
 
 NSRange RangeForCharacterAtIndex(NSString* text, NSUInteger index) {
-  if (text == nil || index >= text.length) {
+  if (text == nil || index > text.length) {
     return NSMakeRange(NSNotFound, 0);
   }
   if (index < text.length)

fml/platform/darwin/string_range_sanitization_unittests.mm

@@ -28,3 +28,8 @@ TEST(StringRangeSanitizationTest, CanHandleUnicodeRange) {
   EXPECT_EQ(result.location, 0UL);
   EXPECT_EQ(result.length, 0UL);
 }
+
+TEST(StringRangeSanitizationTest, HandlesEndOfRange) {
+  EXPECT_EQ(fml::RangeForCharacterAtIndex(@"1234", 4).location, 4UL);
+  EXPECT_EQ(fml::RangeForCharacterAtIndex(@"1234", 4).length, 0UL);
+}

shell/platform/darwin/ios/framework/Source/FlutterTextInputPlugin.mm

@@ -312,7 +312,13 @@ static UIReturnKeyType ToUIReturnKeyType(NSString* inputType) {
 }
 
 - (NSString*)textInRange:(UITextRange*)range {
+  if (!range) {
+    return nil;
+  }
+  NSAssert([range isKindOfClass:[FlutterTextRange class]],
+           @"Expected a FlutterTextRange for range (got %@).", [range class]);
   NSRange textRange = ((FlutterTextRange*)range).range;
+  NSAssert(textRange.location != NSNotFound, @"Expected a valid text range.");
   return [self.text substringWithRange:textRange];
 }
 

shell/platform/darwin/ios/framework/Source/accessibility_text_entry.mm

@@ -23,7 +23,13 @@
 }
 
 - (NSString*)textInRange:(UITextRange*)range {
+  if (!range) {
+    return nil;
+  }
+  NSAssert([range isKindOfClass:[FlutterTextRange class]],
+           @"Expected a FlutterTextRange for range (got %@).", [range class]);
   NSRange textRange = ((FlutterTextRange*)range).range;
+  NSAssert(textRange.location != NSNotFound, @"Expected a valid text range.");
   return [self.text substringWithRange:textRange];
 }