Merge pull request #27587 from fengyuentau:4x/core/filestorage_json_support_backslash

core: support parsing back slash \ in parseKey in FileStorage (JSON) #27587

Fixes #27585 

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [ ] The feature is well documented and sample code can be built with the project CMake

modules/core/src/persistence_json.cpp

@@ -419,10 +419,19 @@ public:
             CV_PARSE_ERROR_CPP( "Key must start with \'\"\'" );
 
         char * beg = ptr + 1;
-
+        std::string key_name;
         do {
-            ++ptr;
-            CV_PERSISTENCE_CHECK_END_OF_BUFFER_BUG_CPP();
+            if (*ptr == '\\') { // skip the next character if current is back slash
+                ++ptr;
+                CV_PERSISTENCE_CHECK_END_OF_BUFFER_BUG_CPP();
+                key_name += *ptr;
+                ++ptr;
+                CV_PERSISTENCE_CHECK_END_OF_BUFFER_BUG_CPP();
+            } else {
+                ++ptr;
+                CV_PERSISTENCE_CHECK_END_OF_BUFFER_BUG_CPP();
+                if (*ptr != '\\' && *ptr != '"') key_name += *ptr;
+            }
         } while( cv_isprint(*ptr) && *ptr != '"' );
 
         if( *ptr != '"' )
@@ -430,7 +439,7 @@ public:
 
         if( ptr == beg )
             CV_PARSE_ERROR_CPP( "Key is empty" );
-        value_placeholder = fs->addNode(collection, std::string(beg, (size_t)(ptr - beg)), FileNode::NONE);
+        value_placeholder = fs->addNode(collection, key_name, FileNode::NONE);
 
         ptr++;
         ptr = skipSpaces( ptr );