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

Program.cs

@@ -88,9 +88,13 @@ AnsiConsole.Write(infoTable);
 AnsiConsole.WriteLine();
 
 // ── Build the chat client with tool-calling support ─────────────────────
+var httpClient = new HttpClient();
+OpenRouterHeaders.ApplyTo(httpClient);
+
 var openAiClient = new OpenAIClient(new ApiKeyCredential(apiKey), new OpenAIClientOptions
 {
-    Endpoint = new Uri(endpoint)
+    Endpoint = new Uri(endpoint),
+    Transport = new System.ClientModel.Primitives.HttpClientPipelineTransport(httpClient)
 });
 
 IChatClient innerClient = openAiClient.GetChatClient(model).AsIChatClient();