fix(ai): fix memory leak of local provider in error paths

Add missing unref calls for local_provider in _ai_request_thread
error handling to prevent memory leaks.

src/ai/ai_client.c

@@ -1434,6 +1434,7 @@ _ai_request_thread(gpointer data)
                                                       local_provider_name, local_provider_name);
         log_error("AI request failed for %s/%s: %s", local_provider_name, local_model, error_msg);
         _aiwin_display_error(user_data, error_msg);
+        ai_provider_unref(local_provider);
         ai_session_unref(session);
         g_free(args);
         return NULL;
@@ -1445,6 +1446,7 @@ _ai_request_thread(gpointer data)
         log_error("AI request failed for %s/%s: Failed to initialize curl",
                   local_provider_name, local_model);
         _aiwin_display_error(user_data, "Failed to initialize curl.");
+        ai_provider_unref(local_provider);
         ai_session_unref(session);
         g_free(args);
         return NULL;