fix: update image models, handle upstream errors in HTTP 200

Three issues found in activity logs:

1. Image model IDs stale — FLUX and Seedream removed from OpenRouter.
   Updated to current models: Gemini 2.5/3.1 Flash Image, Gemini 3 Pro
   Image, GPT-5 Image/Mini. Default changed to gemini-2.5-flash-image.

2. openrouter/free router wraps upstream 429 errors in HTTP 200 body
   with error object. Both text and image response parsers now detect
   this pattern and surface a proper RuntimeException.

3. Image request used modalities: ["image"] but Gemini needs
   ["image", "text"]. Updated to include both.

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

includes/providers/class-open-router-image-provider.php

@@ -233,10 +233,12 @@ class PRAutoBlogger_OpenRouter_Image_Provider implements PRAutoBlogger_Image_Pro
 			'messages'   => [
 				[
 					'role'    => 'user',
-					'content' => $prompt,
+					'content' => 'Generate an image: ' . $prompt,
 				],
 			],
-			'modalities'   => [ 'image' ],
+			// Use ["image", "text"] — some providers (Gemini) require both
+			// modalities even when we only want the image output.
+			'modalities'   => [ 'image', 'text' ],
 			'image_config' => [
 				'aspect_ratio' => $aspect,
 			],