Refactors ml_candidate component structure

What does this MR do and why?

Refactors the component ml/components/ml_candidate.vue into ml/routes/candidates/show/ml_candidates_show.vue, keeping it in the same pattern as the other components. This MR is a pure refactor, no functional or UI changes.

How to set up and validate locally

1. Enable the feature flag

   echo "Feature.enable(:ml_experiment_tracking)" | bundle exec rails c

2. Create an Experiment, Candidate and Metadata (use project id for a chosen project):

   exp = Ml::Experiment.create!(name: "Gitlab Experiment", user_id: user_id, project_id: project_id)
   c = exp.candidates.create!(user_id: user_id, start_time: 0) }
   5.times { |i| c.metadata.create!(name: "metadata#{i}", value: "content#{i}") }
   c.metrics.create!(name: "auc", value: 0.1 , tracked_at: Time.zone.now, step: 1)
   c.metrics.create!(name: "accuracy", value: 0.1 , tracked_at: Time.zone.now, step: 1)
   c.params.create!(name: "algorithm", value: "DecisionTree" )
   puts(exp.candidates[0].iid)

3. Navigate to <your project>/-/ml/candidates/<candidate_iid>

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.

Related to #395035 (closed)