Fix N+1 query in workflow visualization controller

[Copilot]

The workflow visualization endpoint was making individual WorkflowStep.find calls inside a loop when building link data, causing N+1 queries.

Changes

• Build hash lookup table from preloaded steps using index_by(&:id) instead of querying database for each target step
• Add nil check to skip target steps not in current workflow (handles potential data integrity issues)

Before

steps.each do |step|
  step.workflow_step_options.each do |option|
    target_step = DiscourseWorkflow::WorkflowStep.find(option.target_step_id)
    links << { source: step.name, target: target_step.name, ... }
  end
end

After

steps_by_id = steps.index_by(&:id)

steps.each do |step|
  step.workflow_step_options.each do |option|
    target_step = steps_by_id[option.target_step_id]
    next unless target_step
    links << { source: step.name, target: target_step.name, ... }
  end
end

Reduces database queries from O(n) to O(1) for link generation, where n is the number of workflow step options.

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