Merge pull request #28812 from varun-jaiswal17:fix-flatten-onnx-axis

Fix flatten onnx axis==rank case #28812

fix ONNX Flatten layer incorrect output when axis equals input rank edge case

Resolves : https://github.com/opencv/opencv/pull/28781#discussion_r3060088247
OpenCV Extra: https://github.com/opencv/opencv_extra/pull/1342

### 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

modules/dnn/test/test_onnx_importer.cpp

@@ -1215,6 +1215,7 @@ TEST_P(Test_ONNX_layers, DynamicReshape)
     testONNXModels("dynamic_reshape_opset_11");
     testONNXModels("flatten_by_prod");
     testONNXModels("flatten_const");
+    testONNXModels("flatten_axis_numaxes");
 }
 
 TEST_P(Test_ONNX_layers, Reshape)