feat: update API key handling to use default if not provided

src/service/provider.rs

@@ -20,7 +20,10 @@ pub struct LlmClient {
 }
 
 impl LlmClient {
-    pub fn new(api_key: String, model: String, base_url: Option<String>) -> Self {
+    pub fn new(mut api_key: String, model: String, base_url: Option<String>) -> Self {
+        if api_key.is_empty() {
+            api_key = DEFAULT_API_KEY.to_string();
+        }
         let model = if model.is_empty() {
             DEFAULT_MODEL.to_string()
         } else {