branch-4.0: [fix](search) Validate mode parameter in search() DSL options #60785 (#60813)

Cherry-picked from #60785

Co-authored-by: Jack <jiangkai@selectdb.com>

Author: github-actions[bot]

fe/fe-core/src/main/java/org/apache/doris/nereids/trees/expressions/functions/scalar/SearchDslParser.java

@@ -1779,12 +1779,22 @@ public boolean isCrossFieldsMode() {
         /**
          * Validate the options after deserialization.
          * Checks for:
+         * - mode is "standard" or "lucene"
          * - Mutual exclusion between fields and default_field
          * - minimum_should_match is non-negative if specified
          *
          * @throws IllegalArgumentException if validation fails
          */
         public void validate() {
+            // Validation: mode must be "standard" or "lucene"
+            if (mode != null) {
+                String normalizedMode = mode.trim().toLowerCase();
+                if (!"standard".equals(normalizedMode) && !"lucene".equals(normalizedMode)) {
+                    throw new IllegalArgumentException(
+                            "'mode' must be 'standard' or 'lucene', got: " + mode);
+                }
+                this.mode = normalizedMode;
+            }
             // Validation: fields and default_field are mutually exclusive
             if (fields != null && !fields.isEmpty()
                     && defaultField != null && !defaultField.isEmpty()) {

fe/fe-core/src/test/java/org/apache/doris/nereids/trees/expressions/functions/scalar/SearchDslParserTest.java

@@ -1301,6 +1301,18 @@ public void testMultiFieldInvalidType() {
         });
     }
 
+    @Test
+    public void testInvalidMode() {
+        // Test: invalid mode value should throw exception
+        String dsl = "hello";
+        String options = "{\"default_field\":\"title\",\"mode\":\"lucenedfafa\"}";
+
+        IllegalArgumentException exception = Assertions.assertThrows(IllegalArgumentException.class, () -> {
+            SearchDslParser.parseDsl(dsl, options);
+        });
+        Assertions.assertTrue(exception.getMessage().contains("'mode' must be 'standard' or 'lucene'"));
+    }
+
     @Test
     public void testMultiFieldSingleTermSameResultForBothTypes() {
         // Test: single term should have same structure for both types