feat(image-gen): support Codex reference images

[toughCSB]

Summary

• Add input_images support to the openai-codex image generation provider
• Accept local image files, remote image URLs, and data URLs as Responses input_image parts
• Surface invalid local image paths/MIME types as argument errors

Test plan

• python -m py_compile plugins/image_gen/openai-codex/__init__.py tools/image_generation_tool.py tests/plugins/image_gen/test_openai_codex_provider.py tests/tools/test_image_generation_plugin_dispatch.py
• python -m pytest tests/plugins/image_gen/test_openai_codex_provider.py tests/tools/test_image_generation_plugin_dispatch.py -q

Local result: 25 passed.

[kshitijk4poor]

Closing as a duplicate — Codex OAuth image-to-image / reference-image input is now on main via #57017 (merge commit 019950560).

That PR wires source/reference images through the Codex Responses input_image path on the unified image_generate surface, with magic-byte validation, a 25MB cap, the agent.file_safety read guard, and a 16-image clamp. Your PR tackled the same feature — thank you for the contribution and for helping surface how much demand there was for this. Credit to everyone in the cluster who pushed on it.

If you spot a gap in the merged implementation, please open a follow-up. 🙏