[fix](fe) Preserve Paimon CAST predicate semantics on branch-4.1

fe/fe-core/src/main/java/org/apache/doris/datasource/paimon/source/PaimonPredicateConverter.java

@@ -185,15 +185,12 @@ private int getFieldIndex(String colName) {
 
 
     public static SlotRef convertDorisExprToSlotRef(Expr expr) {
-        SlotRef slotRef = null;
+        // Stripping CAST can prune matching rows, e.g. CAST('05' AS INT) = 5 is not '05' = '5'.
+        // Keep casted columns in the original Doris conjuncts to preserve value and null semantics.
         if (expr instanceof SlotRef) {
-            slotRef = (SlotRef) expr;
-        } else if (expr instanceof CastExpr) {
-            if (expr.getChild(0) instanceof SlotRef) {
-                slotRef = (SlotRef) expr.getChild(0);
-            }
+            return (SlotRef) expr;
         }
-        return slotRef;
+        return null;
     }
 
     public LiteralExpr convertDorisExprToLiteralExpr(Expr expr) {

fe/fe-core/src/test/java/org/apache/doris/planner/PaimonPredicateConverterTest.java

@@ -17,6 +17,7 @@
 
 package org.apache.doris.planner;
 
+import org.apache.doris.analysis.CastExpr;
 import org.apache.doris.analysis.Expr;
 import org.apache.doris.common.FeConstants;
 import org.apache.doris.datasource.paimon.source.PaimonPredicateConverter;
@@ -29,10 +30,14 @@
 import org.apache.paimon.predicate.Or;
 import org.apache.paimon.predicate.Predicate;
 import org.apache.paimon.types.DataField;
+import org.apache.paimon.types.DecimalType;
 import org.apache.paimon.types.IntType;
 import org.apache.paimon.types.RowType;
+import org.apache.paimon.types.VarCharType;
 import org.junit.jupiter.api.Assertions;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
 
 import java.util.List;
 
@@ -46,6 +51,57 @@ protected void runBeforeAll() throws Exception {
         String tbl1 = "create table db1.tbl1(" + "k1 int," + " k2 int," + " v1 int)" + " distributed by hash(k1)"
                 + " properties('replication_num' = '1');";
         createTables(tbl1);
+        createTables("create table db1.cast_predicates (id int, code string, amount decimal(10, 2))"
+                + " distributed by hash(id) properties('replication_num' = '1')");
+    }
+
+    @ParameterizedTest
+    @ValueSource(strings = {"cast(code as int) = 5", "5 = cast(code as int)", "cast(code as int) > 5",
+            "cast(code as int) in (5, 6)", "cast(code as int) not in (5, 6)",
+            "cast(code as int) is null", "cast(code as int) is not null",
+            "cast(code as int) = 5 or id = 1"})
+    public void rejectStringToIntegerCast(String predicate) throws Exception {
+        List<Expr> conjuncts = planCastPredicate(predicate);
+        Assertions.assertTrue(conjuncts.stream().anyMatch(expr -> expr.contains(CastExpr.class)), predicate);
+        List<Expr> remaining = Expr.cloneList(conjuncts);
+        Assertions.assertTrue(castPredicateConverter().convertToPaimonExpr(conjuncts).isEmpty(), predicate);
+        Assertions.assertEquals(remaining, conjuncts);
+    }
+
+    @Test
+    public void rejectDecimalScaleCast() throws Exception {
+        List<Expr> conjuncts = planCastPredicate("cast(amount as decimal(10, 1)) = 1.2");
+        Assertions.assertTrue(conjuncts.stream().anyMatch(expr -> expr.contains(CastExpr.class)));
+        Assertions.assertTrue(castPredicateConverter().convertToPaimonExpr(conjuncts).isEmpty());
+    }
+
+    @Test
+    public void retainIndependentUncastPredicates() throws Exception {
+        List<Expr> conjuncts = planCastPredicate("cast(code as int) = 5 and id = 1");
+        List<Expr> remaining = Expr.cloneList(conjuncts);
+        List<Predicate> predicates = castPredicateConverter().convertToPaimonExpr(conjuncts);
+        Assertions.assertEquals(1, predicates.size());
+        Assertions.assertEquals("id", ((LeafPredicate) predicates.get(0)).fieldName());
+        Assertions.assertEquals(remaining, conjuncts);
+
+        for (String predicate : Lists.newArrayList("code = '5'", "id > 1", "id in (1, 2)")) {
+            Assertions.assertEquals(1, castPredicateConverter().convertToPaimonExpr(planCastPredicate(predicate)).size(),
+                    predicate);
+        }
+    }
+
+    private List<Expr> planCastPredicate(String predicate) throws Exception {
+        StmtExecutor executor = new StmtExecutor(connectContext,
+                "select * from db1.cast_predicates where " + predicate);
+        executor.execute();
+        return executor.planner().getScanNodes().get(0).getConjuncts();
+    }
+
+    private PaimonPredicateConverter castPredicateConverter() {
+        return new PaimonPredicateConverter(new RowType(Lists.newArrayList(
+                new DataField(0, "id", new IntType()),
+                new DataField(1, "code", new VarCharType()),
+                new DataField(2, "amount", new DecimalType(10, 2)))));
     }
 
     @Test