Fix RSpec/NamedSubject in ee_root_controller_spec

What does this MR do and why?

Fixes RSpec/NamedSubject offenses in ee/spec/controllers/ee/root_controller_spec.rb by replacing the anonymous subject reference with controller, the RSpec Rails helper method for accessing the controller instance.

The RSpec/NamedSubject cop flags unnamed subjects to keep specs clear and prevent ambiguous subject references.

# bad
RSpec.describe User do
  subject { described_class.new }

  it 'is valid' do
    expect(subject.valid?).to be(true)
  end
end

# good
RSpec.describe User do
  subject(:user) { described_class.new }

  it 'is valid' do
    expect(user.valid?).to be(true)
  end
end

# also good
RSpec.describe User do
  subject(:user) { described_class.new }

  it { is_expected.to be_valid }
end

This file is listed in .rubocop_todo/rspec/named_subject.yml. Its entry has been removed from the todo list as part of this fix.

Offenses fixed

• Line 68: allow(subject).to receive(:current_user) → allow(controller).to receive(:current_user)

References

Relates to: #589138

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