Rebased and updated this PR against the current main after #21273 landed.

What changed in this revision:

• Keeps the existing virtual tier IDs (gpt-image-2-low, gpt-image-2-medium, gpt-image-2-high) as quality selectors.
• Separates those tier IDs from the actual model field sent to images.generate.
• Honors feat(image-gen): honor image_gen.model from config.yaml (salvage #19376) #21273's dispatcher-provided model kwarg:
  • if it is one of the virtual tier IDs, it selects the quality tier;
  • if it is not a virtual tier ID, it is treated as the actual API model for OpenAI-compatible backends.
• Keeps explicit escape hatches with higher precedence:
  • OPENAI_IMAGE_API_MODEL
  • image_gen.openai.api_model
• Adds base URL support through:
  • OPENAI_BASE_URL
  • image_gen.openai.base_url
• Adds tests for API model resolution, base URL resolution, provider.generate(...), and dispatcher model forwarding.

Local verification:

python -m pytest -o addopts= tests/plugins/image_gen/test_openai_provider.py -q
# 37 passed

python -m pytest -o addopts= tests/plugins/image_gen -q
# 74 passed

python -m pytest -o addopts= tests/tools -k image -q
# 187 passed, 7 skipped

python -m ruff check plugins/image_gen/openai/__init__.py tests/plugins/image_gen/test_openai_provider.py tests/tools/test_image_generation_plugin_dispatch.py
# All checks passed

This should make the PR complementary to #21273 rather than conflicting with it: #21273 forwards image_gen.model; this plugin now actually interprets that forwarded value correctly while still supporting provider-specific api_model / base_url config for third-party OpenAI-compatible image endpoints.

CI note after the latest push:

• ruff enforcement (blocking) passed.
• nix ubuntu/macOS, e2e, supply-chain scan, Windows footguns, attribution, and Docker build jobs passed.
• ruff + ty diff failed only while trying to post its PR comment (Resource not accessible by integration on issues/.../comments); the generated summary reported no new ruff issues and labels ty diagnostics as warnings.
• test failed in two unchanged areas outside this PR's diff:
  • tests/gateway/test_restart_drain.py::test_restart_command_while_busy_requests_drain_without_interrupt
  • tests/hermes_cli/test_tencent_tokenhub_provider.py::TestTencentTokenhubContextLength::test_hy3_preview_context_length

This PR's changed files remain limited to:

plugins/image_gen/openai/__init__.py
tests/plugins/image_gen/test_openai_provider.py
tests/tools/test_image_generation_plugin_dispatch.py

Rebased this PR and refreshed CI.

Status after the refresh:

• Head: 87889255b
• Merge state: CLEAN
• Checks: 18 success, 2 skipped

Local verification run before pushing:

uv run --with ruff ruff check plugins/image_gen/openai/__init__.py tests/plugins/image_gen/test_openai_provider.py tests/tools/test_image_generation_plugin_dispatch.py
uv run --extra dev --with pytest-xdist python -m pytest tests/plugins/image_gen/test_openai_provider.py tests/tools/test_image_generation_plugin_dispatch.py tests/gateway/test_restart_drain.py::test_restart_command_while_busy_requests_drain_without_interrupt tests/hermes_cli/test_tencent_tokenhub_provider.py::TestTencentTokenhubContextLength::test_hy3_preview_has_registered_context_length -q --tb=short -n 0

The previous red checks were from the stale branch state; the fork-comment permission issue in the lint summary path is no longer present on the refreshed run.