DEV: Move problem checks to app directory (#26120)

There are a couple of reasons for this.

The first one is practical, and related to eager loading. Since /lib is not eager loaded, when the application boots, ProblemCheck["identifier"] will be nil because the child classes aren't loaded.

The second one is more conceptual. There turns out to be a lot of inter-dependencies between the part of the problem check system that live in /app and the parts that live in /lib, which probably suggests it should all go in /app.

spec/models/problem_check_spec.rb

@@ -0,0 +1,41 @@
+# frozen_string_literal: true
+
+RSpec.describe ProblemCheck do
+  # rubocop:disable RSpec/BeforeAfterAll
+  before(:all) do
+    ScheduledCheck = Class.new(described_class) { self.perform_every = 30.minutes }
+    UnscheduledCheck = Class.new(described_class)
+  end
+
+  after(:all) do
+    Object.send(:remove_const, ScheduledCheck.name)
+    Object.send(:remove_const, UnscheduledCheck.name)
+  end
+  # rubocop:enable RSpec/BeforeAfterAll
+
+  let(:scheduled_check) { ScheduledCheck }
+  let(:unscheduled_check) { UnscheduledCheck }
+
+  describe ".[]" do
+    it { expect(described_class[:scheduled_check]).to eq(scheduled_check) }
+    it { expect(described_class[:foo]).to eq(nil) }
+  end
+
+  describe ".identifier" do
+    it { expect(scheduled_check.identifier).to eq(:scheduled_check) }
+  end
+
+  describe ".checks" do
+    it { expect(described_class.checks).to include(scheduled_check, unscheduled_check) }
+  end
+
+  describe ".scheduled" do
+    it { expect(described_class.scheduled).to include(scheduled_check) }
+    it { expect(described_class.scheduled).not_to include(unscheduled_check) }
+  end
+
+  describe ".scheduled?" do
+    it { expect(scheduled_check).to be_scheduled }
+    it { expect(unscheduled_check).to_not be_scheduled }
+  end
+end