[Spark] Do not use the same variable name in the coordinator registra…

…tion code (#3570)

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#### Which Delta project/connector is this regarding?
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- [X] Spark
- [ ] Standalone
- [ ] Flink
- [ ] Kernel
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## Description

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In commit coordinator registration, we use the same variable name from
the method and case use case which are bad practice. This PR simply
fixes it.


## How was this patch tested?

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Simple variable renaming change + existing unit tests will cover them

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

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No

spark/src/main/scala/org/apache/spark/sql/delta/coordinatedcommits/CommitCoordinatorClient.scala

@@ -57,10 +57,10 @@ object CommitCoordinatorProvider {
   /** Registers a new [[CommitCoordinatorBuilder]] with the [[CommitCoordinatorProvider]] */
   def registerBuilder(commitCoordinatorBuilder: CommitCoordinatorBuilder): Unit = synchronized {
     nameToBuilderMapping.get(commitCoordinatorBuilder.getName) match {
-      case Some(commitCoordinatorBuilder: CommitCoordinatorBuilder) =>
+      case Some(existingBuilder: CommitCoordinatorBuilder) =>
         throw new IllegalArgumentException(
-          s"commit-coordinator: ${commitCoordinatorBuilder.getName} already" +
-          s" registered with builder ${commitCoordinatorBuilder.getClass.getName}")
+          s"commit-coordinator: ${existingBuilder.getName} already" +
+          s" registered with builder ${existingBuilder.getClass.getName}")
       case None =>
         nameToBuilderMapping.put(commitCoordinatorBuilder.getName, commitCoordinatorBuilder)
     }