Don't cast and extract normalization

We are not trying to handle user input so `cast` can be a no-op. Also
extract the base36 normalization logic, let the binary class handle the
serialize encoding and add some tests.

test/lib/rails_ext/active_record_uuid_type_test.rb

@@ -0,0 +1,50 @@
+require "test_helper"
+
+class ActiveRecordUuidTypeTest < ActiveSupport::TestCase
+  setup do
+    @type = ActiveRecord::Type::Uuid.new
+    @sample_uuid = "01jcqzx8h0000000000000000" # base36 UUID
+  end
+
+  test "cast nil returns nil" do
+    assert_nil @type.cast(nil)
+  end
+
+  test "cast returns value as-is" do
+    result = @type.cast(@sample_uuid)
+    assert_equal @sample_uuid, result
+  end
+
+  test "serialize returns binary Data object" do
+    result = @type.serialize(@sample_uuid)
+
+    assert_instance_of ActiveModel::Type::Binary::Data, result
+    assert_equal 16, result.to_s.bytesize
+    assert_equal Encoding::BINARY, result.to_s.encoding
+  end
+
+  test "serialize nil returns nil" do
+    assert_nil @type.serialize(nil)
+  end
+
+  test "deserialize converts binary to base36" do
+    binary_data = @type.serialize(@sample_uuid)
+
+    result = @type.deserialize(binary_data)
+
+    assert_equal @sample_uuid, result
+  end
+
+  test "deserialize handles raw binary string" do
+    binary_data = @type.serialize(@sample_uuid)
+    raw_binary = binary_data.to_s
+
+    result = @type.deserialize(raw_binary)
+
+    assert_equal @sample_uuid, result
+  end
+
+  test "deserialize nil returns nil" do
+    assert_nil @type.deserialize(nil)
+  end
+end