[SPARK-50385][CORE] Use class name prefix for REST Submission API thr…

…ead pool

### What changes were proposed in this pull request?

This PR aims to use a meaningful class name prefix for REST Submission API thread pool instead of the default value of Jetty QueuedThreadPool, `"qtp"+super.hashCode()`.

https://github.com/dekellum/jetty/blob/3dc0120d573816de7d6a83e2d6a97035288bdd4a/jetty-util/src/main/java/org/eclipse/jetty/util/thread/QueuedThreadPool.java#L64

### Why are the changes needed?

This is helpful during JVM investigation.

**BEFORE (4.0.0-preview2)**

```
$ SPARK_MASTER_OPTS='-Dspark.master.rest.enabled=true' sbin/start-master.sh
$ jstack 28217 | grep qtp
"qtp1925630411-52" #52 daemon prio=5 os_prio=31 cpu=0.07ms elapsed=19.06s tid=0x0000000134906c10 nid=0xde03 runnable  [0x0000000314592000]
"qtp1925630411-53" #53 daemon prio=5 os_prio=31 cpu=0.05ms elapsed=19.06s tid=0x0000000134ac6810 nid=0xc603 runnable  [0x000000031479e000]
"qtp1925630411-54" #54 daemon prio=5 os_prio=31 cpu=0.06ms elapsed=19.06s tid=0x000000013491ae10 nid=0xdc03 runnable  [0x00000003149aa000]
"qtp1925630411-55" #55 daemon prio=5 os_prio=31 cpu=0.08ms elapsed=19.06s tid=0x0000000134ac9810 nid=0xc803 runnable  [0x0000000314bb6000]
"qtp1925630411-56" #56 daemon prio=5 os_prio=31 cpu=0.04ms elapsed=19.06s tid=0x0000000134ac9e10 nid=0xda03 runnable  [0x0000000314dc2000]
"qtp1925630411-57" #57 daemon prio=5 os_prio=31 cpu=0.05ms elapsed=19.06s tid=0x0000000134aca410 nid=0xca03 runnable  [0x0000000314fce000]
"qtp1925630411-58" #58 daemon prio=5 os_prio=31 cpu=0.04ms elapsed=19.06s tid=0x0000000134acaa10 nid=0xcb03 runnable  [0x00000003151da000]
"qtp1925630411-59" #59 daemon prio=5 os_prio=31 cpu=0.06ms elapsed=19.06s tid=0x0000000134acb010 nid=0xcc03 runnable  [0x00000003153e6000]
"qtp1925630411-60-acceptor-0108e9815-ServerConnector1e497474{HTTP/1.1, (http/1.1)}{M3-Max.local:6066}" #60 daemon prio=3 os_prio=31 cpu=0.11ms elapsed=19.06s tid=0x00000001317ffa10 nid=0xcd03 runnable  [0x00000003155f2000]
"qtp1925630411-61-acceptor-11d90f2aa-ServerConnector1e497474{HTTP/1.1, (http/1.1)}{M3-Max.local:6066}" #61 daemon prio=3 os_prio=31 cpu=0.10ms elapsed=19.06s tid=0x00000001314ed610 nid=0xcf03 waiting on condition  [0x00000003157fe000]
```

**AFTER**
```
$ SPARK_MASTER_OPTS='-Dspark.master.rest.enabled=true' sbin/start-master.sh
$ jstack 28317 | grep StandaloneRestServer
"StandaloneRestServer-52" #52 daemon prio=5 os_prio=31 cpu=0.09ms elapsed=60.06s tid=0x00000001284a8e10 nid=0xdb03 runnable  [0x000000032cfce000]
"StandaloneRestServer-53" #53 daemon prio=5 os_prio=31 cpu=0.06ms elapsed=60.06s tid=0x00000001284acc10 nid=0xda03 runnable  [0x000000032d1da000]
"StandaloneRestServer-54" #54 daemon prio=5 os_prio=31 cpu=0.05ms elapsed=60.06s tid=0x00000001284ae610 nid=0xd803 runnable  [0x000000032d3e6000]
"StandaloneRestServer-55" #55 daemon prio=5 os_prio=31 cpu=0.09ms elapsed=60.06s tid=0x00000001284aec10 nid=0xd703 runnable  [0x000000032d5f2000]
"StandaloneRestServer-56" #56 daemon prio=5 os_prio=31 cpu=0.06ms elapsed=60.06s tid=0x00000001284af210 nid=0xc803 runnable  [0x000000032d7fe000]
"StandaloneRestServer-57" #57 daemon prio=5 os_prio=31 cpu=0.05ms elapsed=60.06s tid=0x00000001284af810 nid=0xc903 runnable  [0x000000032da0a000]
"StandaloneRestServer-58" #58 daemon prio=5 os_prio=31 cpu=0.06ms elapsed=60.06s tid=0x00000001284afe10 nid=0xcb03 runnable  [0x000000032dc16000]
"StandaloneRestServer-59" #59 daemon prio=5 os_prio=31 cpu=0.05ms elapsed=60.06s tid=0x00000001284b0410 nid=0xcc03 runnable  [0x000000032de22000]
"StandaloneRestServer-60-acceptor-04aefbaa8-ServerConnector44284d85{HTTP/1.1, (http/1.1)}{M3-Max.local:6066}" #60 daemon prio=3 os_prio=31 cpu=0.13ms elapsed=60.05s tid=0x000000015cda1a10 nid=0xcd03 runnable  [0x000000032e02e000]
"StandaloneRestServer-61-acceptor-148976251-ServerConnector44284d85{HTTP/1.1, (http/1.1)}{M3-Max.local:6066}" #61 daemon prio=3 os_prio=31 cpu=0.12ms elapsed=60.05s tid=0x000000015cd1c810 nid=0xce03 waiting on condition  [0x000000032e23a000]
```

### Does this PR introduce _any_ user-facing change?

No, the thread names are accessed during the debugging.

### How was this patch tested?

Manual review.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #48924 from dongjoon-hyun/SPARK-50385.

Authored-by: Dongjoon Hyun <dongjoon@apache.org>
Signed-off-by: panbingkun <panbingkun@apache.org>

core/src/main/scala/org/apache/spark/deploy/rest/RestSubmissionServer.scala

@@ -94,6 +94,7 @@ private[spark] abstract class RestSubmissionServer(
    */
   private def doStart(startPort: Int): (Server, Int) = {
     val threadPool = new QueuedThreadPool(masterConf.get(MASTER_REST_SERVER_MAX_THREADS))
+    threadPool.setName(getClass().getSimpleName())
     if (Utils.isJavaVersionAtLeast21 && masterConf.get(MASTER_REST_SERVER_VIRTUAL_THREADS)) {
       val newVirtualThreadPerTaskExecutor =
         classOf[Executors].getMethod("newVirtualThreadPerTaskExecutor")