feat(image_gen): support openai key_env override

[charzhou]

Summary

• add image_gen.openai.key_env support for the OpenAI image generation provider
• accept image_gen.openai.api_key_env as an alias
• resolve the configured environment variable before falling back to OPENAI_API_KEY
• pass the resolved key explicitly to openai.OpenAI(api_key=...)

Dependency

• This branch is based on the fork main that already includes PR feat(image_gen): support openai base_url override #33647 for image_gen.openai.base_url support.

Tests

• ./.venv/bin/python -m pytest tests/plugins/image_gen/test_openai_provider.py tests/tools/test_image_generation_plugin_dispatch.py tests/agent/test_image_gen_registry.py -q -o addopts=

[charzhou]

Closing to resubmit as a clean feature-isolated PR rebased on the latest upstream main after the fork/upstream merge refresh.