[SPARK-54877][UI] Make display stacktrace on UI error page configurable

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

This PR adds a new configuration `spark.ui.showErrorStacks` to allow user to disable displaying stacktrace on UI error page.

### Why are the changes needed?

To avoid exposing much internal info of the service for security purposes.

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

No, the default value of the newly added configuration does not change the existing behavior.

### How was this patch tested?

Manually tested, modified AllJobsPage.scala to simulate an internal error.

Run with `--conf spark.ui.showErrorStacks=true` (default behavior)

<img width="894" height="756" alt="image" src="https://github.com/user-attachments/assets/cb2e77ef-175a-43b0-bab2-1cb9e294be27" />

Run with `--conf spark.ui.showErrorStacks=false`

<img width="885" height="206" alt="image" src="https://github.com/user-attachments/assets/50d53286-eb11-466e-8c80-52a88a1806e1" />

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

No.

Closes #53656 from pan3793/SPARK-54877.

Authored-by: Cheng Pan <[email protected]>
Signed-off-by: Kousuke Saruta <[email protected]>

Author: pan3793

core/src/main/scala/org/apache/spark/internal/config/UI.scala

@@ -271,4 +271,10 @@ private[spark] object UI {
     .version("4.0.0")
     .timeConf(TimeUnit.MILLISECONDS)
     .createWithDefaultString("30s")
+
+  val UI_SHOW_ERROR_STACKS = ConfigBuilder("spark.ui.showErrorStacks")
+    .doc("Whether to display stack traces in the UI error pages.")
+    .version("4.2.0")
+    .booleanConf
+    .createWithDefault(true)
 }

core/src/main/scala/org/apache/spark/ui/JettyUtils.scala

@@ -265,7 +265,7 @@ private[spark] object JettyUtils extends Logging {
     val server = new Server(pool)
 
     val errorHandler = new ErrorHandler()
-    errorHandler.setShowStacks(true)
+    errorHandler.setShowStacks(conf.get(UI_SHOW_ERROR_STACKS))
     errorHandler.setServer(server)
     server.addBean(errorHandler)