Refine Kotlin Serialization codec type checks

ServerSentEvent and String checks, removed from
KotlinSerializationSupport in Spring Framework 7.0, are reintroduced by
this commit at the right level (KotlinSerializationSupport for
ServerSentEvent and KotlinSerializationString(Decoder|Encoder) for
String).

Closes gh-35885

spring-web/src/main/java/org/springframework/http/codec/KotlinSerializationStringDecoder.java

@@ -102,7 +102,7 @@ public abstract class KotlinSerializationStringDecoder<T extends StringFormat> e
 
 	@Override
 	public boolean canDecode(ResolvableType elementType, @Nullable MimeType mimeType) {
-		return canSerialize(elementType, mimeType);
+		return canSerialize(elementType, mimeType) && !CharSequence.class.isAssignableFrom(elementType.toClass());
 	}
 
 	@Override

spring-web/src/main/java/org/springframework/http/codec/KotlinSerializationStringEncoder.java

@@ -101,7 +101,7 @@ public abstract class KotlinSerializationStringEncoder<T extends StringFormat> e
 
 	@Override
 	public boolean canEncode(ResolvableType elementType, @Nullable MimeType mimeType) {
-		return canSerialize(elementType, mimeType);
+		return canSerialize(elementType, mimeType) && !String.class.isAssignableFrom(elementType.toClass());
 	}
 
 	@Override

spring-web/src/main/java/org/springframework/http/codec/KotlinSerializationSupport.java

@@ -117,7 +117,7 @@ public abstract class KotlinSerializationSupport<T extends SerialFormat> {
 		if (!this.typePredicate.test(type) || ResolvableType.NONE.equals(type)) {
 			return false;
 		}
-		return serializer(type) != null && supports(mimeType);
+		return serializer(type) != null && supports(mimeType) && !ServerSentEvent.class.isAssignableFrom(type.toClass());
 	}
 
 	private boolean supports(@Nullable MimeType mimeType) {

spring-web/src/test/kotlin/org/springframework/http/codec/json/KotlinSerializationJsonDecoderTests.kt

@@ -80,7 +80,7 @@ class KotlinSerializationJsonDecoderTests : AbstractDecoderTests<KotlinSerializa
 		assertThat(decoderForAllTypes.canDecode(ResolvableType.forClass(Pojo::class.java), MediaType.APPLICATION_JSON)).isTrue()
 		assertThat(decoderForAllTypes.canDecode(ResolvableType.forClass(Pojo::class.java), jsonSubtype)).isTrue()
 		assertThat(decoderForAllTypes.canDecode(ResolvableType.forClass(Pojo::class.java), null)).isTrue()
-		assertThat(decoderForAllTypes.canDecode(ResolvableType.forClass(String::class.java), null)).isTrue()
+		assertThat(decoderForAllTypes.canDecode(ResolvableType.forClass(String::class.java), null)).isFalse()
 		assertThat(decoderForAllTypes.canDecode(ResolvableType.forClass(Pojo::class.java), MediaType.APPLICATION_XML)).isFalse()
 		assertThat(decoderForAllTypes.canDecode(ResolvableType.forClass(Pojo::class.java),
 			MediaType("application", "json", StandardCharsets.UTF_8))).isTrue()

spring-web/src/test/kotlin/org/springframework/http/codec/json/KotlinSerializationJsonEncoderTests.kt

@@ -91,7 +91,7 @@ class KotlinSerializationJsonEncoderTests : AbstractEncoderTests<KotlinSerializa
 
 		assertThat(encoderForAllTypes.canEncode(pojoType, MediaType.APPLICATION_NDJSON)).isTrue()
 
-		assertThat(encoderForAllTypes.canEncode(ResolvableType.forClass(String::class.java), null)).isTrue()
+		assertThat(encoderForAllTypes.canEncode(ResolvableType.forClass(String::class.java), null)).isFalse()
 		assertThat(encoderForAllTypes.canEncode(ResolvableType.forClass(Pojo::class.java), MediaType.APPLICATION_XML)).isFalse()
 		val sseType = ResolvableType.forClass(ServerSentEvent::class.java)
 		assertThat(encoderForAllTypes.canEncode(sseType, MediaType.APPLICATION_JSON)).isFalse()