Log when an HTTP error disables GitHub Actions caching

[kwy404]

When the GitHub Actions cache service returns a non-2xx status, #maybeHandleServiceDown sets #serviceIsDown = true and then starts an async task that reads the response body and logs why caching was disabled (rate limit, 503, or another HTTP error). That task first checks if (this.#serviceIsDown) return;, but the flag was set just before, so it always returns and the message is never logged. Network errors take the synchronous branch and are logged fine; only HTTP errors are silent.

The concurrent-request dedup is already handled by the synchronous #serviceIsDown check above, so this removes the extra check inside the async task.

Added a test in cache-github-fake.test.ts that forces a 503 on the first cache lookup and checks that the simple logger prints the "temporarily unavailable" message. It fails before the change and passes after. Also added a CHANGELOG entry under Unreleased > Fixed.

Related to #1330, where the expected behavior was to disable caching for the rest of the run and warn.

[aomarks]

Thanks, looks like this was a regression. I also added a commit to this PR which fixes a related thing that turned up, we were trying to show the error body, but were not.

[kwy404]

Thanks for the review and for the follow up fix!

[kwy404]

Hi @aomarks, friendly ping: this is approved and green. Is there anything else you need from me before it can be merged?