AP-5659: Freeze string literals and use unary + for mutable strings

Handle deprecation notices warning of ruby 4.0 intention to default
string literals to be frozen/immutable.

```shell
/Users/joel.sugarman/github/ministryofjustice/laa-apply-for-legal-aid/app/helpers/hash_format_helper.rb:23:
  warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)
```

app/helpers/hash_format_helper.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module HashFormatHelper
   # These methods add classes to the HTML structure
   def format_hash(hash, html = "")
@@ -19,7 +21,7 @@ def format_hash(hash, html = "")
 
   def build_dl_array(key, value)
     content_tag(:dl, class: "govuk-body kvp govuk-!-margin-bottom-0") do
-      dl_contents = ""
+      dl_contents = +""
       dl_contents << content_tag(:dt, standardize_key(key))
       value.each do |val|
         dl_contents << if val.is_a?(Hash)
@@ -34,7 +36,7 @@ def build_dl_array(key, value)
 
   def build_dl(key, value)
     content_tag(:dl, class: "govuk-body kvp govuk-!-margin-bottom-0") do
-      dl_contents = ""
+      dl_contents = +""
       dl_contents << content_tag(:dt, standardize_key(key))
       dl_contents << if standard_type?(value)
                        content_tag(:dd, value.to_s.capitalize)

app/helpers/language_switcher_helper.rb

@@ -1,6 +1,8 @@
+# frozen_string_literal: true
+
 module LanguageSwitcherHelper
   def language_links
-    links = ""
+    links = +""
 
     I18n.available_locales.each do |locale|
       link = I18n.locale == locale ? t("generic.#{locale}") : govuk_link_to(t("generic.#{locale}"), url_for(locale:))

features/support/steps_helper.rb

@@ -33,7 +33,7 @@
 end
 
 Then("I choose the {string} frequency for {string}") do |frequency, field|
-  parts = [field, "frequency", frequency]
+  parts = [field, +"frequency", frequency]
   parts.each(&:downcase!)
   field_id = parts.join(" ").gsub(/\s+/, "-")
   id = find("input[id*=#{field_id}", visible: false)[:id]