feat: add OpenAI as provider for text and image generation

- Add openai_text.py: text generation via OpenAI chat completions API
  (gpt-4o, gpt-4o-mini, gpt-4.1, gpt-4.1-mini, gpt-4.1-nano, o3-mini)
- Add openai_image.py: image generation via OpenAI images API
  (gpt-image-1 with reference image support, dall-e-3, dall-e-2)
- Refactor builder provider dispatch from TargetType to model-name index
  to support multiple providers per target type
- Fix circular import between config.py and providers/__init__.py
  using TYPE_CHECKING guard
- Fix stale default model assertions in tests
- Add openai>=1.0.0 dependency

pyproject.toml

@@ -9,6 +9,7 @@ dependencies = [
     "httpx>=0.27.0",
     "mistralai>=1.0.0",
     "networkx>=3.6.1",
+    "openai>=1.0.0",
     "pydantic>=2.12.5",
     "pyyaml>=6.0",
     "typer>=0.23.1",