This replaces `assert.True` with `assert.Contains` when testing the
timeout error message in `TestClient_TimeoutIsEnforced`.

This shows the (admittedly lowercased) error message instead of just a
`Should be true` message. This is particularly helpful when it fails in
CI, such as in https://gitlab.com/gitlab-org/labkit/-/jobs/13954002059.

test(httpclient): use better assertion for timeout test

See merge request gitlab-org/labkit!434

Merged-by: Luke Hollinda <lhollinda@gitlab.com>
Approved-by: Luke Hollinda <lhollinda@gitlab.com>
Co-authored-by: Kev Kloss <kkloss@gitlab.com>

Digging through net/http and squinting my eyes, it looks as though even
when our deadline has passed the error we receive is not guaranteed to
contain the word "timeout"

Instead we should try using netErr.Timeout() directly to determine if a
timeout occurred.

Transient provider failures (e.g. Flipt timeout, connection refused)
return a ResolutionError in the BoolResolutionDetail/StringResolutionDetail.
Previously these error results were cached for the full TTL, meaning a
brief outage could poison the cache and prevent recovery until entries
expire.

Check ProviderResolutionDetail.Error() before writing to cache. Only
successful evaluations are now cached; error results always delegate to
the inner provider on the next call.

Ref: https://pkg.go.dev/github.com/open-feature/go-sdk/openfeature#ProviderResolutionDetail.Error

test(httpclient): avoid race condition in TimeoutIsEnforced

See merge request gitlab-org/labkit!435

Merged-by: Doug Barrett <dbarrett@gitlab.com>
Approved-by: Doug Barrett <dbarrett@gitlab.com>
Co-authored-by: Luke Hollinda <lhollinda@gitlab.com>

fix(v2/featureflag): skip caching error evaluation results

See merge request gitlab-org/labkit!436

Merged-by: Matias Alvarez <malvarez@gitlab.com>
Approved-by: Matias Alvarez <malvarez@gitlab.com>
Reviewed-by: GitLab Duo <gitlab-duo@gitlab.com>
Co-authored-by: dbarrett <dbarrett@gitlab.com>