feat: recognize OpenRouter :free models as zero-cost

Models with the :free suffix (e.g. google/gemini-2.5-flash-preview:free)
are served at no charge by OpenRouter. The cost estimator now returns
$0.00 for these models instead of falling through to the conservative
$10/$30 per-million-token fallback.

No changes needed in the model picker or provider — :free models use
the same API endpoint and already appear in the registry.

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

includes/providers/class-open-router-pricing.php

@@ -114,6 +114,11 @@ class PRAutoBlogger_OpenRouter_Pricing {
 	 * @return float Estimated cost in USD.
 	 */
 	public function estimate_cost( string $model, int $prompt_tokens, int $completion_tokens ): float {
+		// OpenRouter `:free` suffix models cost nothing.
+		if ( ':free' === substr( $model, -5 ) ) {
+			return 0.0;
+		}
+
 		$pricing = self::MODEL_PRICING[ $model ] ?? null;
 
 		// Fall back to cached model list pricing.