Fix LLM::CompletionWorker failure of encoding issue

When we are building the diff output to send to the language processing unit, we are running into an issue where we are trying to join UTF-8 and ASCII-8BIT. The error is occuring: https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/gitlab/llm/templates/summarize_merge_request.rb#L43

https://new-sentry.gitlab.net/organizations/gitlab/issues/317664/?project=3&query=is%3Aunresolved++CompletionWorker&referrer=issue-stream&statsPeriod=14d&stream_index=1

https://log.gprd.gitlab.net/app/r/s/QQxbL

It seems to be mainly only one project that this is occurring in.