[KYUUBI #7158] Spark engine respects session-level idle timeout threshold

### Why are the changes needed?
Fixes the same class of issue as
https://github.com/apache/kyuubi/pull/7138

Previously, `sessionIdleTimeoutThreshold` was initialized only once during session creation using `sessionManager.getConf`, preventing dynamic updates when clients pass new configurations during connection.

we now:
- Allow clients to set session-specific kyuubi.session.idle.timeout` during connection
- Dynamically adjust idle timeout per session
- Prevent connection pile-up by timely recycling idle sessions

Closes #7158 from 1358035421/lc/sessio_idle_timeout_threshold.

Closes #7158

abe513eed [liangzhaoyuan] fix review comments
3face844a [liangzhaoyuan] Use per-session idle timeout threshold instead of global sessionManager's value

Authored-by: liangzhaoyuan <lwlzyl19940916@gmail.com>
Signed-off-by: Cheng Pan <chengpan@apache.org>
(cherry picked from commit 9a50bfa814)
Signed-off-by: Cheng Pan <chengpan@apache.org>

externals/kyuubi-spark-sql-engine/src/main/scala/org/apache/kyuubi/engine/spark/session/SparkSessionImpl.scala

@@ -24,6 +24,7 @@ import org.apache.spark.sql.{AnalysisException, SparkSession}
 import org.apache.spark.ui.SparkUIUtils.formatDuration
 
 import org.apache.kyuubi.KyuubiSQLException
+import org.apache.kyuubi.config.KyuubiConf.SESSION_IDLE_TIMEOUT
 import org.apache.kyuubi.config.KyuubiReservedKeys.KYUUBI_SESSION_HANDLE_KEY
 import org.apache.kyuubi.engine.spark.events.SessionEvent
 import org.apache.kyuubi.engine.spark.operation.SparkSQLOperationManager
@@ -57,6 +58,13 @@ class SparkSessionImpl(
     }
   }
 
+  override val sessionIdleTimeoutThreshold: Long = {
+    conf.get(SESSION_IDLE_TIMEOUT.key)
+      .map(_.toLong)
+      .getOrElse(
+        sessionManager.getConf.get(SESSION_IDLE_TIMEOUT))
+  }
+
   private val sessionEvent = SessionEvent(this)
 
   override def open(): Unit = {