Fix duplicate image, remove hardcoded model references

Three fixes in one:

1. Duplicate image: prepend_caption_to_post() was embedding the full
   image + caption as a <figure> block, but the theme already displays
   the featured image. Now only the caption text is prepended — no
   second copy of the image.

2. Hardcoded model names: removed all FLUX.1/FLUX.2 references from
   model-agnostic code (pipeline, prompt builder, sideloader, OpenRouter
   provider, interface, admin labels). The Cloudflare provider still
   references FLUX since it specifically wraps that model family.

3. Test alignment: OpenRouterImagePricingTest was asserting against
   removed models (flux-2-pro, seedream). Updated to match current
   COST_PER_IMAGE entries (Gemini Flash, GPT-5 Image).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

includes/core/class-image-media-sideloader.php

@@ -145,7 +145,7 @@ class PRAutoBlogger_Image_Media_Sideloader {
 			gmdate( 'Y-m-d_H-i-s' )
 		);
 
-		return $base . '.png'; // Default to PNG since FLUX.1 outputs PNG.
+		return $base . '.png'; // Default to PNG — most image models output PNG.
 	}
 
 	/**