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

lib/middleware/discourse_public_exceptions.rb

@@ -4,6 +4,12 @@
 # we need to handle certain exceptions here
 module Middleware
   class DiscoursePublicExceptions < ::ActionDispatch::PublicExceptions
+    # These middlewares will be re-run when the exception response is generated
+    EXCEPTION_RESPONSE_MIDDLEWARES = [
+      ContentSecurityPolicy::Middleware,
+      Middleware::CspScriptNonceInjector,
+    ]
+
     INVALID_REQUEST_ERRORS =
       Set.new(
         [
@@ -60,7 +66,10 @@ module Middleware
           if ApplicationController.rescue_with_handler(exception, object: fake_controller)
             body = response.body
             body = [body] if String === body
-            return response.status, response.headers, body
+            rack_response = [response.status, response.headers, body]
+            app = lambda { |env| rack_response }
+            EXCEPTION_RESPONSE_MIDDLEWARES.each { |middleware| app = middleware.new(app) }
+            return app.call(env)
           end
         rescue => e
           return super if INVALID_REQUEST_ERRORS.include?(e.class)