[CoP] Remove white border from CoP result images

[Haralishev77]

Summary

Remove artificial white borders from Cooperative Perception visualization images by saving the rendered RGB canvas directly instead of embedding it in a Matplotlib figure.

Changes

• Updated the Cooperative Perception visualizer to save canvas arrays directly with plt.imsave().
• Applied the same fix to the upstream OpenCOOD visualization path.
• Preserved the configured DPI and exact canvas dimensions.

Validation

• Simulation run
• Manual verification

Validated that:

• Both modified files compile successfully.
• Saved PNG dimensions exactly match the source canvas.
• Edge pixels are preserved without additional borders..

Checklist

• Kept the PR focused and reviewable
• Updated documentation
• Added or updated validation steps