Add image_edit tool for Codex image editing

[MaxMa04]

Summary

• add a first-class image_edit tool alongside image_generate
• route edit requests through plugin image providers with image_url, optional mask_url, and action=edit
• extend the OpenAI Codex image provider to send GPT Image 2 edit requests via Responses image_generation with input_image
• support local input images by converting them to data URLs before dispatch
• document image editing and add tests for schema/request-shape/mask forwarding

Closes #29956

Tests

• python -m pytest tests/plugins/image_gen/test_openai_codex_provider.py tests/tools/test_image_generation.py tests/agent/test_image_gen_registry.py -q -o 'addopts='

[janeza2]

we need this

[teknium1]

Thanks for the Codex image-editing implementation.

Automated hermes-sweeper review found that the requested capability is now implemented on main through the unified image-generation interface:

• tools/image_generation_tool.py:1175 exposes image_url and reference_image_urls on image_generate and routes source-image requests to editing.
• plugins/image_gen/openai-codex/__init__.py:460 advertises Codex image-input support; :525 normalizes source images before the Responses request is made.
• ecffd290a (feat(image-gen): support Codex image inputs) and 460235d58 are on current main and shipped in v2026.7.7.
• The linked feat(image-gen): support Codex image inputs #55828 discussion records the maintainer confirmation that Codex OAuth image-to-image/reference editing was merged via feat(image-gen): support Codex image inputs (salvage #55828) #57017.

The current design deliberately keeps this as one image_generate surface rather than adding a second model-facing tool, so this PR is redundant.