test: add SLT tests for levenshtein expression

Add SQL logic test file covering:
- Column arguments with NULL propagation
- Column + literal combinations
- Literal + literal edge cases
- Identical string comparison
- Unicode character-level distance

Author: yusinnmao

spark/src/test/resources/sql-tests/expressions/string/levenshtein.sql

@@ -0,0 +1,46 @@
+-- Licensed to the Apache Software Foundation (ASF) under one
+-- or more contributor license agreements.  See the NOTICE file
+-- distributed with this work for additional information
+-- regarding copyright ownership.  The ASF licenses this file
+-- to you under the Apache License, Version 2.0 (the
+-- "License"); you may not use this file except in compliance
+-- with the License.  You may obtain a copy of the License at
+--
+--   http://www.apache.org/licenses/LICENSE-2.0
+--
+-- Unless required by applicable law or agreed to in writing,
+-- software distributed under the License is distributed on an
+-- "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+-- KIND, either express or implied.  See the License for the
+-- specific language governing permissions and limitations
+-- under the License.
+
+statement
+CREATE TABLE test_levenshtein(s1 string, s2 string) USING parquet
+
+statement
+INSERT INTO test_levenshtein VALUES ('kitten', 'sitting'), ('frog', 'fog'), ('abc', 'abc'), ('', 'hello'), ('hello', ''), ('', ''), (NULL, 'test'), ('hello', NULL), (NULL, NULL)
+
+-- column arguments
+query
+SELECT levenshtein(s1, s2) FROM test_levenshtein
+
+-- column + literal
+query
+SELECT levenshtein(s1, 'abc') FROM test_levenshtein
+
+-- literal + column
+query
+SELECT levenshtein('kitten', s2) FROM test_levenshtein
+
+-- literal + literal
+query
+SELECT levenshtein('kitten', 'sitting'), levenshtein('frog', 'fog'), levenshtein('', ''), levenshtein(NULL, 'a')
+
+-- identical strings
+query
+SELECT levenshtein(s1, s1) FROM test_levenshtein
+
+-- unicode characters
+query
+SELECT levenshtein('café', 'cafe'), levenshtein('你好', '你坏')