Fixes : #26811

Updated api_refinement.py to refine the return type of cv2.imread and cv2.imdecode.

• Previously, these functions were typed to always return a valid image.
• Now, they are correctly annotated as MatLike | None, ensuring static type checkers like mypy detect cases where the function might return None.

import tempfile
import cv2

with tempfile.TemporaryFile() as temp_file:
    temp_file.write(b"Corrupted binary data. Not a image")
    temp_file.seek(0)
    image = cv2.imread(str(temp_file.name))
    print(image.shape)

mypy test.py
# output : Success: no issues found in 1 source file

test.py:8: error: Item "None" of "ndarray[Any, dtype[...]] | None" has no attribute "shape"  [union-attr]
Found 1 error in 1 file (checked 1 source file)

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  Patch to opencv_extra has the same branch name.
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