Merge pull request #26846 from MaximSmolskiy:fix_bug_with_int64_support_for_FileStorage

Fix bug with int64 support for FileStorage #26846

### Pull Request Readiness Checklist

Fix #26829, opencv/opencv-python#1078

In current implementation of `int64` support raw size of recorded integer is variable (`4` or `8` bytes depending on value). But then we iterate over nodes we need to know it exact value
https://github.com/opencv/opencv/blob/dfad11aae7ef3b3a0643379266bc363b1a9c3d40/modules/core/src/persistence.cpp#L2596-L2609

Bug is that `rawSize` method still return `4` for any integer. I haven't figured out a way how to get variable raw size for integer in this method. I made raw size for integer is constant and equal to `8`.

Yes, after this patch memory consumption for integers will increase, but I don't know a better way to do it yet. At least this fixes bug and implementation becomes more correct

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.
- [x] The feature is well documented and sample code can be built with the project CMake

Author: MaximSmolskiy

modules/core/src/persistence.cpp

@@ -2441,7 +2441,7 @@ size_t FileNode::rawSize() const
         p += 4;
     size_t sz0 = (size_t)(p - p0);
     if( tp == INT )
-        return sz0 + 4;
+        return sz0 + 8;
     if( tp == REAL )
         return sz0 + 8;
     if( tp == NONE )
@@ -2475,13 +2475,7 @@ void FileNode::setValue( int type, const void* value, int len )
         sz += 4;
 
     if( type == INT )
-    {
-        int64_t ival = *(const int64_t*)value;
-        if (ival > INT_MAX || ival < INT_MIN)
-            sz += 8;
-        else
-            sz += 4;
-    }
+        sz += 8;
     else if( type == REAL )
         sz += 8;
     else if( type == STRING )
@@ -2502,10 +2496,7 @@ void FileNode::setValue( int type, const void* value, int len )
     if( type == INT )
     {
         int64_t ival = *(const int64_t*)value;
-        if (sz > 8)
-            writeInt(p, ival);
-        else
-            writeInt(p, static_cast<int>(ival));
+        writeInt(p, ival);
     }
     else if( type == REAL )
     {

modules/core/test/test_io.cpp

@@ -2003,6 +2003,64 @@ TEST(Core_InputOutput, FileStorage_invalid_attribute_value_regression_25946)
     ASSERT_EQ(0, std::remove(fileName.c_str()));
 }
 
+// see https://github.com/opencv/opencv/issues/26829
+TEST(Core_InputOutput, FileStorage_int64_26829)
+{
+    String content =
+        "%YAML:1.0\n"
+        "String1: string1\n"
+        "IntMin: -2147483648\n"
+        "String2: string2\n"
+        "Int64Min: -9223372036854775808\n"
+        "String3: string3\n"
+        "IntMax: 2147483647\n"
+        "String4: string4\n"
+        "Int64Max: 9223372036854775807\n"
+        "String5: string5\n";
+
+    FileStorage fs(content, FileStorage::READ | FileStorage::MEMORY);
+
+    {
+        std::string str;
+
+        fs["String1"] >> str;
+        EXPECT_EQ(str, "string1");
+
+        fs["String2"] >> str;
+        EXPECT_EQ(str, "string2");
+
+        fs["String3"] >> str;
+        EXPECT_EQ(str, "string3");
+
+        fs["String4"] >> str;
+        EXPECT_EQ(str, "string4");
+
+        fs["String5"] >> str;
+        EXPECT_EQ(str, "string5");
+    }
+
+    {
+        int value;
+
+        fs["IntMin"] >> value;
+        EXPECT_EQ(value, INT_MIN);
+
+        fs["IntMax"] >> value;
+        EXPECT_EQ(value, INT_MAX);
+    }
+
+
+    {
+        int64_t value;
+
+        fs["Int64Min"] >> value;
+        EXPECT_EQ(value, INT64_MIN);
+
+        fs["Int64Max"] >> value;
+        EXPECT_EQ(value, INT64_MAX);
+    }
+}
+
 template <typename T>
 T fsWriteRead(const T& expectedValue, const char* ext)
 {