chore: fix error serialization for string errors

[martingregoire]

When adding custom validation errors, Rails allows both Symbols and Strings:

class Foo < ActiveRecord::Base
  validate :add_symbol_error
  validate :add_string_error

  def add_symbol_error
    errors.add(:title, :too_short)
  end

  def add_string_error
    errors.add(:title, 'is too short')
  end
end

The Symbol error will later be translated by Rails using the i18n logic, when calling foo.errors.full_messages or similar methods. The String error will stay as it is.

see https://api.rubyonrails.org/v7.0.8.7/classes/ActiveModel/Errors.html#method-i-add

This commit introduces support for these String errors in the error serialization class.

Before this change, the error response for the validation that was added in the specs looked like this:

[
  {
    "status"=>"422",
    "source"=>{"pointer"=>"/data/attributes/title"},
    "title"=>"Unprocessable Entity",
    "detail"=> "Title translation missing: en.activerecord.errors.models.note.attributes.title.is not so nice",
    "code"=>"is_not_so_nice"
  }
]

Please note the "translation missing" part in the detail, and the code that is a parameterized version of the String error.

After this change, the error response looks like this:

[
  {
    "status"=>"422",
    "source"=>{"pointer"=>"/data/attributes/title"},
    "title"=>"Unprocessable Entity",
    "detail"=>"Title is not so nice",
    "code"=>"invalid"
  }
]

The detail is now the String error as passed in by Rails, and the code is a generic "invalid" one.