Present pipeline coverage in pipeline API entity as string

Also fixed existing test gap that should have
failed when `Pipeline#coverage` method changed.

Changelog: fixed
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/76299

lib/api/entities/ci/pipeline.rb

@@ -10,7 +10,9 @@ module API
         expose :created_at, :updated_at, :started_at, :finished_at, :committed_at
         expose :duration
         expose :queued_duration
-        expose :coverage
+        expose :coverage do |pipeline|
+          pipeline.present.coverage
+        end
         expose :detailed_status, using: DetailedStatusEntity do |pipeline, options|
           pipeline.detailed_status(options[:current_user])
         end

spec/lib/api/entities/ci/pipeline_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe API::Entities::Ci::Pipeline do
+  let_it_be(:pipeline) { create(:ci_empty_pipeline) }
+  let_it_be(:job) { create(:ci_build, name: "rspec", coverage: 30.212, pipeline: pipeline) }
+
+  let(:entity) { described_class.new(pipeline) }
+
+  subject { entity.as_json }
+
+  it 'returns the coverage as a string' do
+    expect(subject[:coverage]).to eq '30.21'
+  end
+end

spec/requests/api/ci/pipelines_spec.rb

@@ -840,7 +840,7 @@ RSpec.describe API::Ci::Pipelines do
         it 'exposes the coverage' do
           get api("/projects/#{project.id}/pipelines/#{pipeline.id}", user)
 
-          expect(json_response["coverage"].to_i).to eq(30)
+          expect(json_response["coverage"]).to eq('30.00')
         end
       end
     end