fix(conversions): convert BGR/BGRA to gray before thresholding in GrayToBinary

[Jayant-kernel]

Description

Fixes GrayToBinary silently producing a 3-channel output when given a colour (BGR or BGRA) image instead of a grayscale one.

Previously, cv2.threshold ran on each colour channel independently, producing a 3-channel result that was incorrectly labelled as binary. Now the operator converts BGR/BGRA input to grayscale first — consistent with how ColorToBinary handles the same case.

Fixes #319

Type of Change

• Bug fix

How Has This Been Tested?

• New tests added — tests/operators/conversions/test_gray_to_binary.py
  • grayscale input works as before
  • BGR input is converted to gray before thresholding (single-channel output)
  • BGRA input is converted to gray before thresholding (single-channel output)
  • default params produce correct binary output
• Existing tests pass

Checklist

• My code follows the project's style guidelines
• I have performed a self-review
• My changes generate no new warnings
• Tests pass locally