FIX: Improve handling of 'PublicExceptions' when bootstrap_error_pages enabled (#26700)

- Run the CSP-nonce-related middlewares on the generated response

- Fix the readonly mode checking to avoid empty strings being passed (the `check_readonly_mode` before_action will not execute in the case of these re-dispatched exceptions)

- Move the BlockRequestsMiddleware cookie-setting to the middleware, so that it is included even for unusual HTML responses like these exceptions

spec/system/bootstrap_error_pages_spec.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+describe "bootstrap_error_pages", type: :system do
+  before { SiteSetting.bootstrap_error_pages = true }
+
+  it "boots ember for non-existent route" do
+    visit "/foobar"
+    expect(page).not_to have_css("body.no-ember")
+    expect(page).to have_css("#site-logo")
+    expect(page).to have_css("div.page-not-found")
+  end
+
+  it "boots ember for non-existent topic" do
+    visit "/t/999999999999"
+    expect(page).not_to have_css("body.no-ember")
+    expect(page).to have_css("#site-logo")
+    expect(page).to have_css("div.page-not-found")
+  end
+end