Create runner instructions fixtures

What does this MR do and why?

Replace manually created mock data with generated fixtures for runner registration instructions.

Screenshots or screen recordings

NA

How to set up and validate locally

1. Generate the fixtures:

bin/rspec spec/frontend/fixtures/runner_instructions.rb

2. Run the frontend spec:

yarn jest spec/frontend/vue_shared/components/runner_instructions/runner_instructions_modal_spec.js

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.