fix: support 1-channel or 16-bit images in Visualizer

[tybulewicz]

What does this PR do?

This PR addresses Visualizer failures when given grayscale (1-channel) or higher-bit-depth (notably 16-bit) images by normalizing inputs into an RGB 8-bit-compatible form before scene generation and rendering, aligning with the behavior expected by downstream PIL operations (e.g., blending).

Fixes #596

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

[Copilot]

Pull request overview

This PR addresses Visualizer failures when given grayscale (1-channel) or higher-bit-depth (notably 16-bit) images by normalizing inputs into an RGB 8-bit-compatible form before scene generation and rendering, aligning with the behavior expected by downstream PIL operations (e.g., blending).

Changes:

• Add Visualizer._to_rgb() to convert grayscale inputs to RGB and scale 16-bit / float inputs to 8-bit prior to rendering/showing/saving.
• Update Visualizer.show(), save(), and render() to consistently route all inputs through _to_rgb().
• Add extensive unit tests covering grayscale, explicit single-channel arrays, 16-bit arrays/PIL images, and float input scaling.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 6 comments.

File	Description
model_api/src/model_api/visualizer/visualizer.py	Adds _to_rgb() normalization and applies it to show/save/render to support grayscale + 16-bit inputs.
model_api/tests/unit/visualizer/test_visualizer.py	Adds test coverage for grayscale and 16-bit image handling, plus related blending/stacking expectations.

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