feat: Implement OpenRouter API client for managing headers and retrieving model pricing.

OpenRouter/PricingProvider.cs

@@ -14,6 +14,11 @@ internal sealed class PricingProvider
     private static readonly HttpClient Http = new();
     private Dictionary<string, ModelInfo>? _models;
 
+    static PricingProvider()
+    {
+        OpenRouterHeaders.ApplyTo(Http);
+    }
+
     /// <summary>
     /// Fetches the full model list from OpenRouter (cached after first call).
     /// </summary>