[KYUUBI #7XXX] Always update engineError to clear pending reasons

### Why are the changes needed?

In the previous commit [KYUUBI #7313], when a k8s application is pending, we audit the app diagnostics to record the pending reason. This information is eventually updated to the `engineError` field in the metadata store via the `updateMetadata` method.

However, when the application transitions from pending to finished successfully, if there is no error (i.e., `engineError` is `None`), the old pending reason remains in the metadata, causing confusion.

### What are the changes?

Modified `JDBCMetadataStore.updateMetadata()` to always update the `engineError` field:
- Previously: Only updated when `engineError` was `Some(error)`
- Now: Always updates, setting to `null` when `engineError` is `None`

This ensures that pending reasons are properly cleared when the application state changes.

### How was this patch tested?

- Added unit test `update engineError to null when it becomes None` in `JDBCMetadataStoreSuite`
- The test verifies that:
  1. Setting engineError to a pending reason works correctly
  2. Updating engineError to None clears the value in the database

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: turboFei

kyuubi-server/src/main/scala/org/apache/kyuubi/server/metadata/jdbc/JDBCMetadataStore.scala

@@ -376,10 +376,9 @@ class JDBCMetadataStore(conf: KyuubiConf) extends MetadataStore with Logging {
       setClauses += "engine_state = ?"
       params += metadata.engineState
     }
-    metadata.engineError.foreach { error =>
-      setClauses += "engine_error = ?"
-      params += error
-    }
+    // Always update engineError to clear pending reasons when state changes
+    setClauses += "engine_error = ?"
+    params += metadata.engineError.orNull
     if (metadata.peerInstanceClosed) {
       setClauses += "peer_instance_closed = ?"
       params += metadata.peerInstanceClosed

kyuubi-server/src/test/scala/org/apache/kyuubi/server/metadata/jdbc/JDBCMetadataStoreSuite.scala

@@ -271,6 +271,50 @@ class JDBCMetadataStoreSuite extends KyuubiFunSuite {
     assert(jdbcMetadataStore.getLatestSchemaUrl(Seq(url1, url2, url3, url4, url5)).get === url5)
   }
 
+  test("update engineError to null when it becomes None") {
+    val batchId = UUID.randomUUID().toString
+    val kyuubiInstance = "localhost:10099"
+    val batchMetadata = Metadata(
+      identifier = batchId,
+      sessionType = SessionType.BATCH,
+      realUser = "kyuubi",
+      username = "kyuubi",
+      ipAddress = "127.0.0.1",
+      kyuubiInstance = kyuubiInstance,
+      state = "PENDING",
+      resource = "intern",
+      className = "org.apache.kyuubi.SparkWC",
+      requestName = "kyuubi_batch",
+      requestConf = Map("spark.master" -> "local"),
+      requestArgs = Seq("100"),
+      createTime = System.currentTimeMillis(),
+      engineType = "spark",
+      clusterManager = Some("local"))
+
+    jdbcMetadataStore.insertMetadata(batchMetadata)
+
+    // Simulate k8s app pending with error message
+    val pendingMetadata = batchMetadata.copy(
+      state = "PENDING",
+      engineError = Some("Pod pending: Insufficient CPU"))
+    jdbcMetadataStore.updateMetadata(pendingMetadata)
+
+    var retrievedMetadata = jdbcMetadataStore.getMetadata(batchId)
+    assert(retrievedMetadata.engineError == Some("Pod pending: Insufficient CPU"))
+
+    // When app transitions to success, engineError should be cleared to None/null
+    val runningMetadata = pendingMetadata.copy(
+      state = "RUNNING",
+      engineError = None)
+    jdbcMetadataStore.updateMetadata(runningMetadata)
+
+    retrievedMetadata = jdbcMetadataStore.getMetadata(batchId)
+    assert(retrievedMetadata.engineError == None)
+
+    // Clean up
+    jdbcMetadataStore.cleanupMetadataByIdentifier(batchId)
+  }
+
   test("kubernetes engine info") {
     val tag = UUID.randomUUID().toString
     val metadata = KubernetesEngineInfo(