[http_request] Call container->end() on non-OK status, fix comment

Address Copilot review feedback:
- Call container->end() when status_code != HTTP_STATUS_OK (pre-existing
  bug, but easy to fix while we're here)
- Update error_str comment to say "update check failure" not "fetch failure"

esphome/components/http_request/update/http_request_update.cpp

@@ -85,6 +85,8 @@ void HttpRequestUpdate::update_task(void *params) {
 
   if (container == nullptr || container->status_code != HTTP_STATUS_OK) {
     ESP_LOGE(TAG, "Failed to fetch manifest from %s", this_update->source_url_.c_str());
+    if (container != nullptr)
+      container->end();
     info->error_str = LOG_STR("Failed to fetch manifest");
     goto defer;  // NOLINT(cppcoreguidelines-avoid-goto)
   }

esphome/components/update/update_entity.h

@@ -16,7 +16,7 @@ struct UpdateInfo {
   std::string release_url;
   std::string firmware_url;
   std::string md5;
-  const LogString *error_str{nullptr};  // Set on fetch failure, nullptr on success
+  const LogString *error_str{nullptr};  // Set on update check failure, nullptr on success
   bool has_progress{false};
   float progress;
 };