[SPARK-54358][SDP][FOLLOWUP] Add tableIdentifierToPathString helper method for SDP checkpoint path construction

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

Followups from #53070 to improve code clarity.

### Why are the changes needed?

Make sure the code for constructing SDP checkpoint directory paths is clear.

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

No

### How was this patch tested?

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

Closes #53089 from sryza/collide-followups.

Authored-by: Sandy Ryza <sandy.ryza@databricks.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
(cherry picked from commit 10d1b4c)
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

Author: sryza

sql/pipelines/src/main/scala/org/apache/spark/sql/pipelines/graph/SystemMetadata.scala

@@ -22,6 +22,7 @@ import scala.util.Try
 import org.apache.hadoop.fs.Path
 
 import org.apache.spark.internal.{Logging, LogKeys}
+import org.apache.spark.sql.catalyst.TableIdentifier
 import org.apache.spark.sql.classic.SparkSession
 
 sealed trait SystemMetadata {}
@@ -44,7 +45,9 @@ case class FlowSystemMetadata(
     Option(if (graph.table.contains(flow.destinationIdentifier) ||
       graph.sink.contains(flow.destinationIdentifier)) {
       val checkpointRoot = new Path(context.storageRoot, "_checkpoints")
-      val flowTableId = flow.destinationIdentifier.nameParts.mkString(Path.SEPARATOR)
+      // Different tables in the pipeline can have flows with the same name, so we include
+      // the table's fully qualified identifier in the path to avoid collisions.
+      val flowTableId = tableIdentifierToPathString(flow.destinationIdentifier)
       val flowName = flow.identifier.table
       val checkpointDir = new Path(
         new Path(checkpointRoot, flowTableId),
@@ -62,6 +65,13 @@ case class FlowSystemMetadata(
     })
   }
 
+  /**
+   * Converts a TableIdentifier to a path string by joining its name parts with the path separator.
+   */
+  private def tableIdentifierToPathString(tableIdentifier: TableIdentifier): String = {
+    tableIdentifier.nameParts.mkString(Path.SEPARATOR)
+  }
+
   /** Returns the location for the most recent checkpoint of a given flow. */
   def latestCheckpointLocation: String = {
     val checkpointsDir = flowCheckpointsDirOpt().get