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

bulkgen/providers/__init__.py

@@ -4,10 +4,13 @@ from __future__ import annotations
 
 import abc
 from pathlib import Path
+from typing import TYPE_CHECKING
 
-from bulkgen.config import TargetConfig
 from bulkgen.providers.models import ModelInfo
 
+if TYPE_CHECKING:
+    from bulkgen.config import TargetConfig
+
 
 class Provider(abc.ABC):
     """Abstract base for generation providers."""