RESTEasy (Reactive) JSON serialization for multipart form data containing emojis is wrong
### Describe the bug

Serialization (string) data containing emojis as JSON form parts in multipart responses or payloads differs from serialization in non-multipart contexts:

```
@Path("/test")
public class ReactiveMultipartTestResource {

    public static class TestData {
        public String bar;

        public TestData(String bar) {
            this.bar = bar;
        }
    }

    public static class FormData {
        @RestForm
        @PartType(MediaType.APPLICATION_JSON)
        public TestData data;
        @RestForm
        public String dataAsString;
    }

    @Inject
    ObjectMapper om;

    @GET
    @Produces(MediaType.MULTIPART_FORM_DATA)
    public FormData hello() throws JsonProcessingException {
        FormData form = new FormData();
        form.data = new TestData("Bar äöü 😀");
        form.dataAsString = om.writeValueAsString(form.data);
        return form;
    }
}
```

Looking at the raw request's response shows different encodings for the emoji part. It's not a "simple" UTF-8 problem as the German umlauts for example are correct. This is taken from IntelliJ's HTTP request tool's response file, not sure how encoding is screwed by pasting and submitting here though...)

```
--b7ae92fe-eb4e-4a8b-b142-c77b79adbd88
Content-Disposition: form-data; name="data"
Content-Type: application/json

{"bar":"Bar Ã¤Ã¶Ã¼ \\uD83D\\uDE00"}
--b7ae92fe-eb4e-4a8b-b142-c77b79adbd88
Content-Disposition: form-data; name="dataAsString"
Content-Type: text/plain

{"bar":"Bar Ã¤Ã¶Ã¼ ð"}
--b7ae92fe-eb4e-4a8b-b142-c77b79adbd88--
```


### Expected behavior

Serialized JSON should be the same for "standard" serialization and "automatic" serialization in multipart contexts.

### Actual behavior

See above :)

### How to Reproduce?

See above :)

### Output of `uname -a` or `ver`

_No response_

### Output of `java -version`

_No response_

### GraalVM version (if different from Java)

_No response_

### Quarkus version or git rev

2.7.1

### Build tool (ie. output of `mvnw --version` or `gradlew --version`)

_No response_

### Additional information

_No response_
diff --git a/independent-projects/resteasy-reactive/server/jackson/src/main/java/org/jboss/resteasy/reactive/server/jackson/JacksonMessageBodyWriterUtil.java b/independent-projects/resteasy-reactive/server/jackson/src/main/java/org/jboss/resteasy/reactive/server/jackson/JacksonMessageBodyWriterUtil.java
index 9de661d986f..d9bbd9fc5c9 100644
--- a/independent-projects/resteasy-reactive/server/jackson/src/main/java/org/jboss/resteasy/reactive/server/jackson/JacksonMessageBodyWriterUtil.java
+++ b/independent-projects/resteasy-reactive/server/jackson/src/main/java/org/jboss/resteasy/reactive/server/jackson/JacksonMessageBodyWriterUtil.java
@@ -55,7 +55,7 @@ public static void doLegacyWrite(Object o, Annotation[] annotations, Multivalued
                     }
                 }
             }
-            entityStream.write(defaultWriter.writeValueAsBytes(o));
+            entityStream.write(defaultWriter.writeValueAsString(o).getBytes(StandardCharsets.UTF_8));
         }
     }
 
['independent-projects/resteasy-reactive/server/jackson/src/main/java/org/jboss/resteasy/reactive/server/jackson/JacksonMessageBodyWriterUtil.java']