Provide type information for optional thread dump endpoint response fields

Closes gh-15989

spring-boot-project/spring-boot-actuator-autoconfigure/src/test/java/org/springframework/boot/actuate/autoconfigure/endpoint/web/documentation/ThreadDumpEndpointDocumentationTests.java

@@ -84,20 +84,21 @@ public class ThreadDumpEndpointDocumentationTests
 										.description(
 												"Description of the object on which the "
 														+ "thread is blocked, if any.")
-										.optional(),
-								fieldWithPath("threads.[].lockInfo").description(
+										.optional().type(JsonFieldType.STRING),
+								fieldWithPath("threads.[].lockInfo")
+										.description(
 												"Object for which the thread is blocked "
 														+ "waiting.")
-										.optional(),
+										.optional().type(JsonFieldType.OBJECT),
 								fieldWithPath("threads.[].lockInfo.className")
 										.description(
 												"Fully qualified class name of the lock"
 														+ " object.")
-										.optional(),
+										.optional().type(JsonFieldType.STRING),
 								fieldWithPath("threads.[].lockInfo.identityHashCode")
 										.description(
 												"Identity hash code of the lock object.")
-										.optional(),
+										.optional().type(JsonFieldType.NUMBER),
 								fieldWithPath("threads.[].lockedMonitors").description(
 										"Monitors locked by this thread, if any"),
 								fieldWithPath("threads.[].lockedMonitors.[].className")