Fix frozen string in validatable, use multiline string instead.

Expand tests to check for the actual validatable exception message

This was raising a `FrozenError` on Ruby < 3 where interpolated strings
were considered frozen. This [changed in Ruby 3], since such strings are
dynamic there's no point in freezing them by default.

The test wasn't catching this because `FrozenError` actually inherits
from `RuntimeError`:

>> FrozenError.ancestors
=> [FrozenError, RuntimeError, StandardError, Exception, Object ...]

So the exception check passed. Now we're also checking for the error
message to ensure it raised the exception we really expected there.

Closes #5465

[changed in Ruby 3] https://bugs.ruby-lang.org/issues/17104

CHANGELOG.md

@@ -1,5 +1,7 @@
 ### Unreleased
 
+* bug fixes
+  * Fix frozen string exception in validatable. [#5563](https://github.com/heartcombo/devise/pull/5563) [@mameier](https://github.com/mameier)
 
 ### 4.9.0 - 2023-02-17
 

lib/devise/models/validatable.rb

@@ -47,7 +47,7 @@ def self.assert_validations_api!(base) #:nodoc:
         unavailable_validations = VALIDATIONS.select { |v| !base.respond_to?(v) }
 
         unless unavailable_validations.empty?
-          raise "Could not use :validatable module since #{base} does not respond " <<
+          raise "Could not use :validatable module since #{base} does not respond " \
                 "to the following methods: #{unavailable_validations.to_sentence}."
         end
       end

test/models/validatable_test.rb

@@ -110,9 +110,12 @@ class ValidatableTest < ActiveSupport::TestCase
   end
 
   test 'should not be included in objects with invalid API' do
-    assert_raise RuntimeError do
+    exception = assert_raise RuntimeError do
       Class.new.send :include, Devise::Models::Validatable
     end
+
+    expected_message = /Could not use :validatable module since .* does not respond to the following methods: validates_presence_of.*/
+    assert_match expected_message, exception.message
   end
 
   test 'required_fields should be an empty array' do