Merge pull request #25810 from fengyuentau:python/fix_parsing_3d_mat_in_dnn

python: attempts to fix 3d mat parsing problem for dnn #25810

Fixes https://github.com/opencv/opencv/issues/25762 https://github.com/opencv/opencv/issues/23242
Relates https://github.com/opencv/opencv/issues/25763 https://github.com/opencv/opencv/issues/19091

Although `cv.Mat` has already been introduced to workaround this problem, people do not know it and it kind of leads to confusion with `numpy.array`. This patch adds a "switch" to turn off the auto multichannel feature when the API is from cv::dnn::Net (more specifically, `setInput`) and the parameter is of type `Mat`. This patch only leads to changes of three places in `pyopencv_generated_types_content.h`:

```.diff
static PyObject* pyopencv_cv_dnn_dnn_Net_setInput(PyObject* self, PyObject* py_args, PyObject* kw)
{
...
- pyopencv_to_safe(pyobj_blob, blob, ArgInfo("blob", 0)) &&
+ pyopencv_to_safe(pyobj_blob, blob, ArgInfo("blob", 8)) &&
...
}

// I guess we also need to change this as one-channel blob is expected for param
static PyObject* pyopencv_cv_dnn_dnn_Net_setParam(PyObject* self, PyObject* py_args, PyObject* kw)
{
...
- pyopencv_to_safe(pyobj_blob, blob, ArgInfo("blob", 0)) )
+ pyopencv_to_safe(pyobj_blob, blob, ArgInfo("blob", 8)) )
...
- pyopencv_to_safe(pyobj_blob, blob, ArgInfo("blob", 0)) )
+ pyopencv_to_safe(pyobj_blob, blob, ArgInfo("blob", 8)) )
...
}
```

Others are unchanged, e.g. `dnn_SegmentationModel` and stuff like that.

### 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/python/src2/cv2.hpp

@@ -46,19 +46,22 @@ private:
     static const uint32_t arg_outputarg_flag     = 0x1;
     static const uint32_t arg_arithm_op_src_flag = 0x2;
     static const uint32_t arg_pathlike_flag      = 0x4;
+    static const uint32_t arg_nd_mat_flag        = 0x8;
 
 public:
     const char* name;
     bool outputarg;
     bool arithm_op_src;
     bool pathlike;
+    bool nd_mat;
     // more fields may be added if necessary
 
     ArgInfo(const char* name_, uint32_t arg_) :
         name(name_),
         outputarg((arg_ & arg_outputarg_flag) != 0),
         arithm_op_src((arg_ & arg_arithm_op_src_flag) != 0),
-        pathlike((arg_ & arg_pathlike_flag) != 0) {}
+        pathlike((arg_ & arg_pathlike_flag) != 0),
+        nd_mat((arg_ & arg_nd_mat_flag) != 0) {}
 
 private:
     ArgInfo(const ArgInfo&) = delete;