[fix](fe) Restrict multi distinct count variant check

### What problem does this PR solve?

Issue Number: close #25672

Related PR: None

Problem Summary: The previous variant fix made MultiDistinctCount reuse the full COUNT DISTINCT argument check, which also rejects complex types. Existing P0 cases use multi_distinct_count on ARRAY values, so that broader check caused regressions. Keep the existing COUNT DISTINCT complex-type validation for Count, but make MultiDistinctCount only apply the new VARIANT rejection.

### Release note

None

### Check List (For Author)

- Test: Unit Test
    - `./run-fe-ut.sh --run org.apache.doris.nereids.trees.expressions.functions.agg.CountTest`
- Behavior changed: No
- Does this need documentation: No

Author: eldenmoon

fe/fe-core/src/main/java/org/apache/doris/nereids/trees/expressions/functions/agg/Count.java

@@ -99,15 +99,20 @@ public void checkLegalityAfterRewrite() {
     }
 
     static void checkDistinctArgument(Expression argument, String functionSql) {
+        checkVariantArgument(argument, functionSql);
+        DataType argumentType = argument.getDataType();
+        if (argumentType.isComplexType() || argumentType.isObjectType() || argumentType.isJsonType()) {
+            throw new AnalysisException("COUNT DISTINCT could not process type " + functionSql);
+        }
+    }
+
+    static void checkVariantArgument(Expression argument, String functionSql) {
         DataType argumentType = argument.getDataType();
         if (argumentType.isVariantType()) {
             throw new AnalysisException("COUNT DISTINCT does not support VARIANT argument in " + functionSql
                     + ". Cast the VARIANT expression to STRING or another supported scalar type before using "
                     + "COUNT DISTINCT.");
         }
-        if (argumentType.isComplexType() || argumentType.isObjectType() || argumentType.isJsonType()) {
-            throw new AnalysisException("COUNT DISTINCT could not process type " + functionSql);
-        }
     }
 
     public boolean isStar() {

fe/fe-core/src/main/java/org/apache/doris/nereids/trees/expressions/functions/agg/MultiDistinctCount.java

@@ -58,7 +58,7 @@ private MultiDistinctCount(boolean distinct, List<Expression> children) {
             throw new AnalysisException("MultiDistinctCount's children size must be 1");
         }
         for (Expression argument : super.children()) {
-            Count.checkDistinctArgument(argument, "COUNT DISTINCT " + argument.toSql());
+            Count.checkVariantArgument(argument, "COUNT DISTINCT " + argument.toSql());
         }
     }
 
@@ -71,7 +71,7 @@ protected MultiDistinctCount(AggregateFunctionParams functionParams) {
     public MultiDistinctCount withDistinctAndChildren(boolean distinct, List<Expression> children) {
         Preconditions.checkArgument(children.size() == 1, "MultiDistinctCount's children size must be 1");
         for (Expression argument : children) {
-            Count.checkDistinctArgument(argument, "COUNT DISTINCT " + argument.toSql());
+            Count.checkVariantArgument(argument, "COUNT DISTINCT " + argument.toSql());
         }
         return new MultiDistinctCount(getFunctionParams(false, children));
     }

fe/fe-core/src/test/java/org/apache/doris/nereids/trees/expressions/functions/agg/CountTest.java

@@ -19,6 +19,8 @@
 
 import org.apache.doris.nereids.exceptions.AnalysisException;
 import org.apache.doris.nereids.trees.expressions.SlotReference;
+import org.apache.doris.nereids.types.ArrayType;
+import org.apache.doris.nereids.types.IntegerType;
 import org.apache.doris.nereids.types.VariantType;
 
 import org.junit.jupiter.api.Assertions;
@@ -42,4 +44,10 @@ void testMultiDistinctCountRejectsVariant() {
         Assertions.assertTrue(exception.getMessage().contains("COUNT DISTINCT does not support VARIANT argument"));
         Assertions.assertTrue(exception.getMessage().contains("Cast the VARIANT expression"));
     }
+
+    @Test
+    void testMultiDistinctCountAllowsArray() {
+        Assertions.assertDoesNotThrow(
+                () -> new MultiDistinctCount(SlotReference.of("arr", ArrayType.of(IntegerType.INSTANCE))));
+    }
 }