test(nodes): add pixel conversion tests for chroma subsampling boundaries

[staging-devin-ai-integration]

Context

PR #505 added I420/NV12 round-trip tests with solid colors (white, black, red). These avoid chroma subsampling edge cases because every 2×2 block has identical RGB values.

Suggested change

Add tests with:

• Mixed-color images (e.g., checkerboard pattern) to exercise per-2×2 chroma averaging
• Odd dimensions (e.g., 5×5, 7×3) to test boundary column/row handling in rgba8_to_i420_buf and rgba8_to_nv12_buf

Origin

Identified by automated review on #505.