Merge pull request #26326 from gursimarsingh:object_detection_fixed

[BUG FIX] Object detection sample preprocessing #26326

PR resloves #26315 related to incorrect preprocessing for 'Image2BlobParams' in object detection sample.
### 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.
- [x] The feature is well documented and sample code can be built with the project CMake

samples/dnn/object_detection.cpp

@@ -399,7 +399,7 @@ void preprocess(const Mat& frame, Net& net, Size inpSize)
     else{
         //![preprocess_call]
         Image2BlobParams imgParams(
-            scale,
+            Scalar::all(scale),
             size,
             meanv,
             swapRB,
@@ -603,7 +603,7 @@ void postprocess(Mat& frame, const vector<Mat>& outs, Net& net, int backend, vec
         }
         if (framework == "onnx"){
             Image2BlobParams paramNet;
-                paramNet.scalefactor = scale;
+                paramNet.scalefactor = Scalar::all(scale);
                 paramNet.size = Size(inpWidth, inpHeight);
                 paramNet.mean = meanv;
                 paramNet.swapRB = swapRB;