Fix epic creation in LLM specs

What does this MR do and why?

Epic state_id enum declaration has been removed so we must use the integer value.

See !141954 (merged)

Example failure: https://gitlab.com/gitlab-org/gitlab/-/jobs/6002324186

You can test locally with:

bin/rspec /ee/spec/lib/gitlab/llm/chain/agents/zero_shot/qa_evaluation_spec.rb

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