Return unmapped timezones from `country_zones`

If a country doesn't exist in the MAPPINGS hash then create a new
`ActiveSupport::Timezone` instance using the supplied timezone id.

Fixes #28431.

activesupport/lib/active_support/values/time_zone.rb

@@ -250,14 +250,21 @@ module ActiveSupport
       # for time zones in the country specified by its ISO 3166-1 Alpha2 code.
       def country_zones(country_code)
         code = country_code.to_s.upcase
-        @country_zones[code] ||=
-          TZInfo::Country.get(code).zone_identifiers.map do |tz_id|
-            name = MAPPING.key(tz_id)
-            name && self[name]
-          end.compact.sort!
+        @country_zones[code] ||= load_country_zones(code)
       end
 
       private
+        def load_country_zones(code)
+          country = TZInfo::Country.get(code)
+          country.zone_identifiers.map do |tz_id|
+            if MAPPING.value?(tz_id)
+              self[MAPPING.key(tz_id)]
+            else
+              create(tz_id, nil, TZInfo::Timezone.new(tz_id))
+            end
+          end.sort!
+        end
+
         def zones_map
           @zones_map ||= begin
             MAPPING.each_key { |place| self[place] } # load all the zones

activesupport/test/time_zone_test.rb

@@ -714,6 +714,10 @@ class TimeZoneTest < ActiveSupport::TestCase
     assert_not_includes ActiveSupport::TimeZone.country_zones(:ru), ActiveSupport::TimeZone["Kuala Lumpur"]
   end
 
+  def test_country_zones_without_mappings
+    assert_includes ActiveSupport::TimeZone.country_zones(:sv), ActiveSupport::TimeZone["America/El_Salvador"]
+  end
+
   def test_to_yaml
     assert_equal("--- !ruby/object:ActiveSupport::TimeZone\nname: Pacific/Honolulu\n", ActiveSupport::TimeZone["Hawaii"].to_yaml)
     assert_equal("--- !ruby/object:ActiveSupport::TimeZone\nname: Europe/London\n", ActiveSupport::TimeZone["Europe/London"].to_yaml)