[SPARK-51192][CONNECT] Expose `processWithoutResponseObserverForTesti…

…ng` in `SparkConnectPlanner`

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

Adds a new `processWithoutResponseObserverForTesting` private-package method scoped to `planner`. This method is called by `SparkConnectPlannerTestUtils#transform`.

### Why are the changes needed?

#47816 attempted to move `MockObserver` into source code to address compilation errors when open-source libraries attempt to test their command plugin extensions via the `SparkConnectPlannerUtils`.

However, this isn't enough as the error `java.lang.NoSuchMethodError: 'void org.apache.spark.sql.connect.planner.SparkConnectPlanner.process(org.apache.spark.connect.proto.Command, io.grpc.stub.StreamObserver` continues to be seen.

To address this shading issue, we move the creation of the `MockObserver` to the source code.

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

No

### How was this patch tested?

Existing tests

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

No.

Closes #49921 from vicennial/SPARK-51192.

Authored-by: vicennial <venkata.gudesa@databricks.com>
Signed-off-by: Herman van Hovell <herman@databricks.com>

sql/connect/server/src/main/scala/org/apache/spark/sql/connect/planner/SparkConnectPlanner.scala

@@ -2474,6 +2474,15 @@ class SparkConnectPlanner(
     }
   }
 
+  /**
+   * Exposed for testing. Processes a command without a response observer.
+   *
+   * Called only from SparkConnectPlannerTestUtils.
+   */
+  private[planner] def processWithoutResponseObserverForTesting(command: proto.Command): Unit = {
+    process(command, new MockObserver())
+  }
+
   def process(
       command: proto.Command,
       responseObserver: StreamObserver[ExecutePlanResponse]): Unit = {

sql/connect/server/src/test/scala/org/apache/spark/sql/connect/planner/SparkConnectPlannerTestUtils.scala

@@ -31,7 +31,7 @@ object SparkConnectPlannerTestUtils {
 
   def transform(spark: SparkSession, command: proto.Command): Unit = {
     val executeHolder = buildExecutePlanHolder(spark, command)
-    new SparkConnectPlanner(executeHolder).process(command, new MockObserver())
+    new SparkConnectPlanner(executeHolder).processWithoutResponseObserverForTesting(command)
   }
 
   private def buildExecutePlanHolder(