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-prompt-builder.php

@@ -5,7 +5,7 @@ declare(strict_types=1);
  * Builds image prompts from article content or source data.
  *
  * Uses a cheap LLM (via OpenRouter) to distill article content into concise
- * visual scene descriptions optimised for FLUX.1. Falls back to rule-based
+ * visual scene descriptions optimised for image generation. Falls back to rule-based
  * synthesis if the LLM call fails, so image generation never blocks on a
  * prompt-rewriting outage.
  *