New unittest case to validate Java parseJson behavior with 1-element arrays.

In Java, it seems like the existing behavior is that we allow
single-element arrays to be parsed into a non-repeated field.

In `public final class JsonArray extends JsonElement implements Iterable<JsonElement>`:
```
  @Override
  public String getAsString() {
    return getAsSingleElement().getAsString();
  }
```

TBH I don't know if this behavior is necessarily _desired_, but here is
a unit test to validate this behavior so that we don't accidentally
break it without intending to.

PiperOrigin-RevId: 947320631

java/util/src/test/java/com/google/protobuf/util/JsonFormatTest.java

@@ -1963,6 +1963,16 @@ public class JsonFormatTest {
     assertThat(actualBuilder.build()).isEqualTo(expected);
   }
 
+  // TODO: b/534418787 - Investigate this behavior further, especially in conformance tests.
+  @Test
+  public void testParserSingleElementArrayForNonRepeatedEnum() throws Exception {
+    TestAllTypes.Builder actualBuilder = TestAllTypes.newBuilder();
+    mergeFromJson("{\"optionalNestedEnum\": [\"FOO\"]}", actualBuilder);
+
+    TestAllTypes expected = TestAllTypes.newBuilder().setOptionalNestedEnum(NestedEnum.FOO).build();
+    assertThat(actualBuilder.build()).isEqualTo(expected);
+  }
+
   @Test
   public void testCustomJsonName() throws Exception {
     TestCustomJsonName message = TestCustomJsonName.newBuilder().setValue(12345).build();