…dius-for-corners-neighbors-in-ChessBoardDetector-findQuadNeighbors

Increase neighbors search radius for corners in ChessBoardDetector:findQuadNeighbors opencv#26014

I didn't do everything right the way I wanted at opencv#25991. I forgot that `edge_len` is edge **squared** length as well as `thresh_scale` is threshold for **squared** scale. So, I wanted to increase scale by `sqrt(2)` times (idea is to use quad diagonal instead of quad side) and therefore `thresh_scale` should be equal to `sqrt(2)^2 = 2`.

And refactor variables names to explicitly indicate that they are squared, so that no one else falls into this trap

I tested this PR with benchmark
```
python3 objdetect_benchmark.py --configuration=generate_run --board_x=7 --path=res_chessboard --synthetic_object=chessboard
```
PR increases detected chessboards number by `1/2%`:
```
cell_img_size = 100 (default)

before
                                 category  detected chessboard  total detected chessboard  total chessboard  average detected error chessboard
                                      all             0.941667                      13560             14400                           0.596726
Total detected time:  136.68963200000007 sec

after
                                 category  detected chessboard  total detected chessboard  total chessboard  average detected error chessboard
                                      all             0.952083                      13710             14400                           0.595984
Total detected time:  136.55770600000014 sec

----------------------------------------------------------------------------------------------------------------------------------------------

cell_img_size = 10

before
                                 category  detected chessboard  total detected chessboard  total chessboard  average detected error chessboard
                                      all             0.579167                       8340             14400                           4.198448
Total detected time:  2.535998999999999 sec

after
                                 category  detected chessboard  total detected chessboard  total chessboard  average detected error 
                                      all             0.591389                       8516             14400                           4.155250
Total detected time:  2.700832999999997 sec
```

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      Patch to opencv_extra has the same branch name.
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