[Vardhman Gupta (Kaap10)]

Closes #3257

Description

BaseImageToImageConverter (which powers ImageResizingConverter) and ImageCompressionConverter support converting and compressing images across JPEG, PNG, and WEBP formats.

However, when processing an image in CMYK color mode (common in print media, scans, and press JPEGs) with output_format="PNG", Pillow's PNG encoder (PIL.PngImagePlugin) throws an uncaught KeyError: 'CMYK' wrapped in OSError: cannot write mode CMYK as PNG because PNG does not support CMYK.

• In BaseImageToImageConverter._transform_image: format preparation was only performed for output_format == "JPEG". When output_format == "PNG", CMYK images were passed directly to transformed.save(buffer, "PNG").
• In ImageCompressionConverter._compress_image: format preparation was also only performed for output_format == "JPEG". PNG targets with CMYK mode bypassed mode normalization and crashed upon image.save(compressed_bytes, output_format, **save_kwargs).

This change normalizes CMYK images to RGB before saving to PNG:

• In BaseImageToImageConverter._transform_image: converts CMYK images using the existing self._convert_to_rgb_or_rgba(image) helper when output_format == "PNG".
• In ImageCompressionConverter._compress_image: converts CMYK images to RGB via image.convert("RGB") when output_format == "PNG".
• Native grayscale (L, LA, 1), paletted (P), and transparent (RGBA) PNGs remain completely untouched.

Tests and Documentation

• Added unit tests:
  • tests/unit/converter/test_image_resizing_converter.py: test_image_resizing_converter_cmyk_to_png_conversion verifies CMYK to PNG conversion, resulting format PNG, mode RGB, and expected resized dimensions.
  • tests/unit/converter/test_image_compression_converter.py: test_image_compression_converter_cmyk_to_png_conversion verifies CMYK compression to PNG without raising OSError, yielding format PNG and mode RGB.
• Verified test suite:
  • uv run python -m pytest tests/unit/converter/test_image_resizing_converter.py tests/unit/converter/test_image_compression_converter.py: 42 passed in 6.40s
  • Sibling image converter tests (saturation, rotation, overlay): 89 passed in 3.21s
• Ran ruff check and ruff format --check clean.

[Vardhman Gupta (Kaap10)]

Normalizes CMYK images to RGB before saving to PNG in BaseImageToImageConverter and ImageCompressionConverter, preventing unhandled Pillow OSError crashes while preserving native grayscale and RGBA modes.

Ready for review!