Fix TypeError when passing an instance of a subclass of String to a string field (#13818)

## Issue
When an object that is an instance of a string-derived class is passed to a string field in a protobuf message in Ruby, it results in a `Google::Protobuf::TypeError`.

### Steps to reproduce
```rb
~/src/github.com/protocolbuffers/protobuf/ruby/tests ❯❯❯ irb -I .
irb(main):001:0> require 'basic_test_pb'
=> true
irb(main):002:0> myString = Class.new(String)
=> #<Class:0x00000001531540d8>
irb(main):003:0> str = myString.new("foo")
=> "foo"
irb(main):004:0> BasicTest::TestMessage.new(optional_string: "foo")
=> <BasicTest::TestMessage: optional_string: "foo", repeated_int32: [], repeated_int64: [], repeated_uint32: [], repeated_uint64: [], repeated_bool: [], repeated_float: [], repeated_double: [], repeated_string: [], repeated_bytes: [], repeated_msg: [], repeated_enum: []>
irb(main):005:0> BasicTest::TestMessage.new(optional_string: str)
(irb):5:in `initialize': Invalid argument for string field 'optional_string' (given #<Class:0x00000001531540d8>). (Google::Protobuf::TypeError)
irb(main):006:0>
```

## Fix
The issue appears to be caused by the field checking mechanism not properly handling instances of classes that inherit from basic types like String. My proposed solution is to improve the type checking for string fields to consider not just String instances but also instances of subclasses of String.

## Impact
The changes will allow instances of classes derived from String to be passed to string fields without any error.
This is a backwards-compatible change and will not affect the existing behaviour with standard String instances.

Closes #13818

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/13818 from NotFounds:support-string-subclass-for-ruby 2d2796c4f9
PiperOrigin-RevId: 590941235

ruby/ext/google/protobuf_c/convert.c

@@ -141,7 +141,7 @@ upb_MessageValue Convert_RubyToUpb(VALUE value, const char* name,
       VALUE utf8 = rb_enc_from_encoding(rb_utf8_encoding());
       if (rb_obj_class(value) == rb_cSymbol) {
         value = rb_funcall(value, rb_intern("to_s"), 0);
-      } else if (rb_obj_class(value) != rb_cString) {
+      } else if (!rb_obj_is_kind_of(value, rb_cString)) {
         rb_raise(cTypeError,
                  "Invalid argument for string field '%s' (given %s).", name,
                  rb_class2name(CLASS_OF(value)));

ruby/tests/basic.rb

@@ -787,4 +787,15 @@ module BasicTest
     assert_respond_to msg, :has_d?
     refute msg.has_d?
   end
+
+  def test_string_subclass
+    str = "hello"
+    myString = Class.new(String)
+
+    m = proto_module::TestMessage.new(
+      optional_string: myString.new(str),
+    )
+
+    assert_equal str, m.optional_string
+  end
 end