Add unit test for new scalar type

spec/graphql/types/duration_type_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe GitlabSchema.types['Duration'] do
+  let(:duration) { 17.minutes }
+
+  it 'presents information as a floating point number' do
+    expect(described_class.coerce_isolated_result(duration)).to eq(duration.to_f)
+  end
+
+  it 'accepts integers as input' do
+    expect(described_class.coerce_isolated_input(100)).to eq(100.0)
+  end
+
+  it 'accepts floats as input' do
+    expect(described_class.coerce_isolated_input(0.5)).to eq(0.5)
+  end
+
+  it 'rejects invalid input' do
+    expect { described_class.coerce_isolated_input('not valid') }
+      .to raise_error(GraphQL::CoercionError)
+  end
+
+  it 'rejects nil' do
+    expect { described_class.coerce_isolated_input(nil) }
+      .to raise_error(GraphQL::CoercionError)
+  end
+end