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-support.php

@@ -59,6 +59,16 @@ class PRAutoBlogger_OpenRouter_Image_Support {
 			);
 		}
 
+		// OpenRouter routers may wrap upstream errors in an HTTP 200 body.
+		if ( isset( $decoded['error']['message'] ) ) {
+			throw new \RuntimeException(
+				sprintf(
+					esc_html__( 'OpenRouter upstream image error: %s', 'prautoblogger' ),
+					esc_html( $decoded['error']['message'] )
+				)
+			);
+		}
+
 		$images = $decoded['choices'][0]['message']['images'] ?? [];
 		if ( empty( $images ) ) {
 			throw new \RuntimeException(