Correctly ignore `case_sensitive` for UUID uniqueness validation

I think we should deprecate this behavior and just error if you tell us
to do a case insensitive comparison for types which are not case
sensitive. Partially reverts 35592307

Fixes #18195

activerecord/lib/active_record/connection_adapters/column.rb

@@ -16,7 +16,7 @@ module Format
       attr_reader :name, :cast_type, :null, :sql_type, :default, :default_function
 
       delegate :type, :precision, :scale, :limit, :klass, :accessor,
-        :number?, :binary?, :changed?,
+        :text?, :number?, :binary?, :changed?,
         :type_cast_from_user, :type_cast_from_database, :type_cast_for_database,
         :type_cast_for_schema,
         to: :cast_type

activerecord/lib/active_record/connection_adapters/postgresql/oid/specialized_string.rb

@@ -8,6 +8,10 @@ class SpecializedString < Type::String # :nodoc:
           def initialize(type)
             @type = type
           end
+
+          def text?
+            false
+          end
         end
       end
     end

activerecord/lib/active_record/type/string.rb

@@ -21,6 +21,10 @@ def type_cast_for_database(value)
         end
       end
 
+      def text?
+        true
+      end
+
       private
 
       def cast_value(value)

activerecord/lib/active_record/type/value.rb

@@ -50,6 +50,10 @@ def type_cast_for_schema(value) # :nodoc:
 
       # These predicates are not documented, as I need to look further into
       # their use, and see if they can be removed entirely.
+      def text? # :nodoc:
+        false
+      end
+
       def number? # :nodoc:
         false
       end

activerecord/lib/active_record/validations/uniqueness.rb

@@ -65,7 +65,7 @@ def build_relation(klass, table, attribute, value) #:nodoc:
           value = value.to_s[0, column.limit]
         end
 
-        if !options[:case_sensitive] && value.is_a?(String)
+        if !options[:case_sensitive] && value && column.text?
           # will use SQL LOWER function before comparison, unless it detects a case insensitive collation
           klass.connection.case_insensitive_comparison(table, attribute, column, value)
         else

activerecord/test/cases/adapters/postgresql/uuid_test.rb

@@ -116,6 +116,23 @@ def test_schema_dump_with_shorthand
     output = dump_table_schema "uuid_data_type"
     assert_match %r{t.uuid "guid"}, output
   end
+
+  def test_uniqueness_validation_ignores_uuid
+    klass = Class.new(ActiveRecord::Base) do
+      self.table_name = "uuid_data_type"
+      validates :guid, uniqueness: { case_sensitive: false }
+
+      def self.name
+        "UUIDType"
+      end
+    end
+
+    record = klass.create!(guid: "a0ee-bc99-9c0b-4ef8-bb6d-6bb9-bd38-0a11")
+    duplicate = klass.new(guid: record.guid)
+
+    assert record.guid.present? # Ensure we actually are testing a UUID
+    assert_not duplicate.valid?
+  end
 end
 
 class PostgresqlLargeKeysTest < ActiveRecord::TestCase