Fix undefined response in reindexing Grafana notifier

What does this MR do and why

notify_start and notify_end discarded the HTTP response and kept only .success?, but the line right after asks response.code:

success = Gitlab::HTTP.post(...).success?

log_error("Response code #{response.code}") unless success

response is not a local variable, not a method on the class and not inherited: the class has no superclass and no include, so the call raises NameError. NameError is a StandardError, so the rescue StandardError right below catches it and logs its message instead. The practical effect is that the Grafana status code is replaced by a generic message precisely when the annotation request failed, which is the moment the line exists to report.

This MR captures the response and derives success from it, so the failure path logs the code it always meant to log.

The spec was encoding the bug

The failure example passed before this change for the wrong reason. Its response double only stubs success?, so response.code raised NameError, the rescue swallowed it, and the example's be_falsey assertion still held. With the fix, the line actually runs and the double receives an unexpected :code.

So the spec is updated rather than merely repaired: it now stubs code and asserts that the failure path logs the status code, which is the behaviour this MR restores. The example fails without the production change and passes with it.

How to verify

The class has ten methods and no ancestor providing response, so the read cannot resolve at runtime. Reading the full class is the check here: a grep for def response is not enough, because an attr_reader would not show up in it.

Spotted by itaruby(to be released), a Ruby type checker I'm working on.