[SPARK-54369][CONNECT][TESTS] Fix PythonPipelineSuite flakiness via Set instead of Seq

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

This PR aims to fix `PythonPipelineSuite` flakiness via `Set` instead of `Seq` in multiple places.

### Why are the changes needed?

Currently, `PythonPipelineSuite` is flaky like the following. We should fix this flakiness.
- https://github.com/apache/spark/actions/runs/19396864076/job/55498096472
```
[info] - referencing internal datasets *** FAILED *** (821 milliseconds)
[info]   List(`spark_catalog`.`default`.`src`, `spark_catalog`.`default`.`c`, `spark_catalog`.`default`.`a`) did not equal List(`spark_catalog`.`default`.`src`, `spark_catalog`.`default`.`a`, `spark_catalog`.`default`.`c`) (PythonPipelineSuite.scala:366)
```

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

No.

### How was this patch tested?

Pass the CIs.

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

No.

Closes #53080 from dongjoon-hyun/SPARK-XXX.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

sql/connect/server/src/test/scala/org/apache/spark/sql/connect/pipelines/PythonPipelineSuite.scala

@@ -364,11 +364,13 @@ class PythonPipelineSuite
 
     val (streamingFlows, batchFlows) = graph.resolvedFlows.partition(_.df.isStreaming)
     assert(
-      batchFlows.map(_.identifier) == Seq(
+      batchFlows.map(_.identifier).toSet == Set(
         graphIdentifier("src"),
         graphIdentifier("a"),
         graphIdentifier("c")))
-    assert(streamingFlows.map(_.identifier) == Seq(graphIdentifier("b"), graphIdentifier("d")))
+    assert(
+      streamingFlows.map(_.identifier).toSet ==
+        Set(graphIdentifier("b"), graphIdentifier("d")))
   }
 
   test("referencing external datasets") {
@@ -722,7 +724,8 @@ class PythonPipelineSuite
     assert(
       graph
         .flowsTo(graphIdentifier("a"))
-        .map(_.identifier) == Seq(graphIdentifier("a"), graphIdentifier("something")))
+        .map(_.identifier)
+        .toSet == Set(graphIdentifier("a"), graphIdentifier("something")))
   }
 
   test("groupby and rollup works with internal datasets, referencing with (col, str)") {