[SPARK-33038][SQL] Combine AQE initial and current plan string when two plans are the same

[allisonwang-db]

What changes were proposed in this pull request?

This PR combines the current plan and the initial plan in the AQE query plan string when the two plans are the same. It also removes the == Current Plan == and == Initial Plan == headers:

Before

AdaptiveSparkPlan isFinalPlan=false
+- == Current Plan ==
   SortMergeJoin [key#13], [a#23], Inner
   :- Sort [key#13 ASC NULLS FIRST], false, 0
   :  +- Exchange hashpartitioning(key#13, 5), true, [id=#94]
            ...
+- == Initial Plan ==
   SortMergeJoin [key#13], [a#23], Inner
   :- Sort [key#13 ASC NULLS FIRST], false, 0
   :  +- Exchange hashpartitioning(key#13, 5), true, [id=#94]
            ...

After

AdaptiveSparkPlan isFinalPlan=false
+- SortMergeJoin [key#13], [a#23], Inner
   :- Sort [key#13 ASC NULLS FIRST], false, 0
   :  +- Exchange hashpartitioning(key#13, 5), true, [id=#94]
            ...

For SQL EXPLAIN output:
Before

AdaptiveSparkPlan (8)
+- == Current Plan ==
   Sort (7)
   +- Exchange (6)
      ...
+- == Initial Plan ==
   Sort (7)
   +- Exchange (6)
      ...

After

AdaptiveSparkPlan (8)
+- Sort (7)
   +- Exchange (6)
      ...

Why are the changes needed?

To simplify the AQE plan string by removing the redundant plan information.

Does this PR introduce any user-facing change?

Yes.

How was this patch tested?

Modified the existing unit test.

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/33900/

[SparkQA]

Kubernetes integration test status success
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/33900/

[SparkQA]

Test build #129284 has finished for PR 29915 at commit fe4873e.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[gatorsmile]

LGTM

[gatorsmile]

Thanks! Merged to master.