feat(image-gen): add opt-in image_edit tool

[BeliefanX]

Summary

• Adds an opt-in image_edit tool under the existing image_gen toolset.
• Dispatches to the configured image provider only when it advertises edit support and exposes edit().
• Preserves ordered multi-image inputs across images, reference_images, and references; duplicates are intentionally preserved.

Relationship

• Layer 4 of the image-edit split: opt-in model-facing image_edit tool surface.
• Logical review order: feat(image-gen): add provider edit contract #26967 → feat(image-gen): validate image references #26968 → feat(image-gen): add OpenAI Codex image edit provider #40491 → feat(image-gen): add opt-in image_edit tool #40498.
• These PRs are intentionally related but not GitHub-stacked; each targets main so CI remains independent.
• Supersedes the tool-surface portion of feat(image-gen): add opt-in image_edit tool #21765, which is closed in favor of this cleaner replacement.
• This PR can be reviewed after the provider/contract pieces; image_edit remains opt-in via image_gen/legacy image_tools, not a core tool.

Testing

• venv/bin/python -m py_compile tools/image_edit_tool.py tests/tools/test_image_edit_tool.py acp_adapter/tools.py model_tools.py tests/acp/test_tools.py toolsets.py
• venv/bin/python -m pytest tests/tools/test_image_edit_tool.py tests/acp/test_tools.py -q -o 'addopts=' — 84 passed
• venv/bin/python -m pytest tests/tools/test_registry.py tests/test_model_tools.py -q -o 'addopts=' — 56 passed
• venv/bin/python -m ruff check tools/image_edit_tool.py tests/tools/test_image_edit_tool.py acp_adapter/tools.py tests/acp/test_tools.py model_tools.py toolsets.py
• git diff --check