feat(image_gen): support openai base_url override

[charzhou]

Summary

• add explicit image_gen.openai.base_url support for the OpenAI image generation provider
• pass the configured endpoint to openai.OpenAI(base_url=...) after normalizing trailing slashes
• keep existing behavior unchanged when no image-gen-specific base URL is configured

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=

[alt-glitch]

Related to open #18796 which adds both OPENAI_IMAGE_API_MODEL env var and base_url config for third-party image-gen backends. This PR focuses narrowly on the base_url override only.