Default to gpt-image-2 for OpenAI image generation

Make gpt-image-2 the default image-generation model and add it to the
recognized model-id mapping, updating the sample provider configs to match.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/sample-provider-configs/openai.yml

@@ -21,7 +21,7 @@ text_to_speech:
   speed: 1.0
   response_format: opus
 image_generation:
-  model_id: gpt-image-1.5
+  model_id: gpt-image-2
   style: null
   size: null
   quality: null