fix: preserve reference image dimensions in server requests

[mikemikimike]

Summary

• Preserve the source dimensions of ref_images, sdapi extra_images, and OpenAI edit uploads during decoding.
• Keep output-size decoding for init and mask images unchanged.
• Add a regression test covering native reference dimensions alongside init/mask resizing.

Issue: #2004
Fixes #2004

Testing

• cmake -S . -B build -DSD_BUILD_EXAMPLES=ON -DSD_BUILD_TESTS=ON -DSD_SERVER_BUILD_FRONTEND=OFF -DSD_WEBP=OFF -DSD_WEBM=OFF -DCMAKE_BUILD_TYPE=Release
• cmake --build build --parallel 4
• ctest --test-dir build --output-on-failure
• sd-cli --help
• sd-server --help