Tighten bound for test_process_pixel_gray

[montplaisir]

Proposed change(s)

Tighten bound for test_process_pixel_gray
Rebase changes from PR #5690 on develop branch
Co-authored-by: @austereantelope

Types of change(s)

• Bug fix
• New feature
• Code refactor
• Breaking change
• Documentation update
• Other (please describe)

Checklist

• Added tests that prove my fix is effective or that my feature works
• Updated the changelog (if applicable)
• Updated the documentation (if applicable)
• Updated the migration guide (if applicable)

[u-pr]

💡 Harness Review

Narrow test-only change; the tighter bound still matches the deterministic quantization error introduced by the PNG round-trip.

_{Reviewed commit 424f5cc8}

_{🤖 Helpful? 👍/👎}