Remove duplicate flushes in HttpMessageConverter implementations

Prior to this commit, a few implementations of the `HttpMessageConverter`
contract were inheriting from abstract classes. Those classes were
performing extra `OutputStream#flush` on the response body even though
this is the responsibility of the super class. Such abstract classes
do flush already, after delegating to the `writeInternal` method.

This commit ensures that we remove such extra calls as they tend to
waste resources for no added benefit.

Closes gh-36383

spring-web/src/main/java/org/springframework/http/converter/ByteArrayHttpMessageConverter.java

@@ -23,7 +23,6 @@ import org.jspecify.annotations.Nullable;
 import org.springframework.http.HttpInputMessage;
 import org.springframework.http.HttpOutputMessage;
 import org.springframework.http.MediaType;
-import org.springframework.util.StreamUtils;
 
 /**
  * Implementation of {@link HttpMessageConverter} that can read and write byte arrays.
@@ -65,7 +64,7 @@ public class ByteArrayHttpMessageConverter extends AbstractHttpMessageConverter<
 
 	@Override
 	protected void writeInternal(byte[] bytes, HttpOutputMessage outputMessage) throws IOException {
-		StreamUtils.copy(bytes, outputMessage.getBody());
+		outputMessage.getBody().write(bytes);
 	}
 
 	@Override

spring-web/src/main/java/org/springframework/http/converter/KotlinSerializationBinaryHttpMessageConverter.java

@@ -91,7 +91,6 @@ public abstract class KotlinSerializationBinaryHttpMessageConverter<T extends Bi
 		try {
 			byte[] bytes = format.encodeToByteArray(serializer, object);
 			outputMessage.getBody().write(bytes);
-			outputMessage.getBody().flush();
 		}
 		catch (SerializationException ex) {
 			throw new HttpMessageNotWritableException("Could not write " + format + ": " + ex.getMessage(), ex);

spring-web/src/main/java/org/springframework/http/converter/KotlinSerializationStringHttpMessageConverter.java

@@ -98,7 +98,6 @@ public abstract class KotlinSerializationStringHttpMessageConverter<T extends St
 			String s = format.encodeToString(serializer, object);
 			Charset charset = charset(outputMessage.getHeaders().getContentType());
 			outputMessage.getBody().write(s.getBytes(charset));
-			outputMessage.getBody().flush();
 		}
 		catch (SerializationException ex) {
 			throw new HttpMessageNotWritableException("Could not write " + format + ": " + ex.getMessage(), ex);

spring-web/src/main/java/org/springframework/http/converter/ResourceHttpMessageConverter.java

@@ -156,7 +156,6 @@ public class ResourceHttpMessageConverter extends AbstractHttpMessageConverter<R
 			try {
 				OutputStream out = outputMessage.getBody();
 				in.transferTo(out);
-				out.flush();
 			}
 			catch (NullPointerException ignored) {
 				// see SPR-13620

spring-web/src/main/java/org/springframework/http/converter/protobuf/ProtobufHttpMessageConverter.java

@@ -237,11 +237,9 @@ public class ProtobufHttpMessageConverter extends AbstractHttpMessageConverter<M
 			OutputStreamWriter outputStreamWriter = new OutputStreamWriter(outputMessage.getBody(), charset);
 			TextFormat.printer().print(message, outputStreamWriter);
 			outputStreamWriter.flush();
-			outputMessage.getBody().flush();
 		}
 		else if (this.protobufFormatDelegate != null) {
 			this.protobufFormatDelegate.print(message, outputMessage, contentType, charset);
-			outputMessage.getBody().flush();
 		}
 	}