Fix SLO violation on llm_completion metric

What does this MR do and why?

This MR fixes SLO violation on llm_completion metric. https://dashboards.gitlab.net/d/sidekiq-main/sidekiq3a-overview?orgId=1&from=now-2d&to=now&viewPanel=3924591239

!135920 (merged) changed the response in Completions::SummarizeMergeRequest to be nil when the diff is too big. This is considered as an error in the Llm::CompletionWorker due to response.try(:errors)&.empty?.

Fixes gitlab-com/gl-infra/production#17102 (closed)

See https://gitlab.slack.com/archives/C064WMDEQE7/p1699496933150329 for more information

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