[SPARK-41391][SQL] The output column name of groupBy.agg(count_distin…

…ct) is incorrect

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

correct the output column name of groupBy.agg(count_distinct),  so the "*" is expanded correctly into column names and the output column has the distinct keyword.

### Why are the changes needed?

Output column name for groupBy.agg(count_distinct)  is incorrect . However similar queries in spark sql return correct output column. For groupBy.agg queries on dataframe "*" is not expanded correctly in the output column  and the distinct keyword is missing from output column.

```
// initializing data
scala> val df = spark.range(1, 10).withColumn("value", lit(1))
df: org.apache.spark.sql.DataFrame = [id: bigint, value: int]
scala> df.createOrReplaceTempView("table")

// Dataframe  aggregate queries with incorrect output column
scala> df.groupBy("id").agg(count_distinct($"*"))
res3: org.apache.spark.sql.DataFrame = [id: bigint, count(unresolvedstar()): bigint]
scala> df.groupBy("id").agg(count_distinct($"value"))
res1: org.apache.spark.sql.DataFrame = [id: bigint, count(value): bigint]

// Spark Sql aggregate queries with correct output column
scala> spark.sql(" SELECT id, COUNT(DISTINCT *) FROM table GROUP BY id ")
res4: org.apache.spark.sql.DataFrame = [id: bigint, count(DISTINCT id, value): bigint]
scala> spark.sql(" SELECT id, COUNT(DISTINCT value) FROM table GROUP BY id ")
res2: org.apache.spark.sql.DataFrame = [id: bigint, count(DISTINCT value): bigint]
```

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

No

### How was this patch tested?

Added UT

Closes apache#40116 from ritikam2/master.

Authored-by: Ritika Maheshwari <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>

sql/core/src/main/scala/org/apache/spark/sql/RelationalGroupedDataset.scala

@@ -89,6 +89,7 @@ class RelationalGroupedDataset protected[sql](
     case expr: NamedExpression => expr
     case a: AggregateExpression if a.aggregateFunction.isInstanceOf[TypedAggregateExpression] =>
       UnresolvedAlias(a, Some(Column.generateAlias))
+    case u: UnresolvedFunction => UnresolvedAlias(expr, None)
     case expr: Expression => Alias(expr, toPrettySQL(expr))()
   }
 

sql/core/src/test/scala/org/apache/spark/sql/DataFrameSuite.scala

@@ -1137,6 +1137,18 @@ class DataFrameSuite extends QueryTest
     checkAnswer(approxSummaryDF, approxSummaryResult)
   }
 
+  test("SPARK-41391: Correct the output column name of groupBy.agg(count_distinct)") {
+    withTempView("person") {
+      person.createOrReplaceTempView("person")
+      val df1 = person.groupBy("id").agg(count_distinct(col("name")))
+      val df2 = spark.sql("SELECT id, COUNT(DISTINCT name) FROM person GROUP BY id")
+      assert(df1.columns === df2.columns)
+      val df3 = person.groupBy("id").agg(count_distinct(col("*")))
+      val df4 = spark.sql("SELECT id, COUNT(DISTINCT *) FROM person GROUP BY id")
+      assert(df3.columns === df4.columns)
+    }
+  }
+
   test("summary advanced") {
     val stats = Array("count", "50.01%", "max", "mean", "min", "25%")
     val orderMatters = person2.summary(stats: _*)