Add support for validating SQL queries with the WITH clause

[zaleslaw]

Fixes #1326

Also I investigated over popular approaches in the supported dialects

To avoid this validation, we have a flag which could be recommended for the user with custom database

[Copilot]

Pull Request Overview

This PR adds support for validating and executing SQL queries that start with WITH, alongside existing SELECT queries, and includes a new integration test using the WITH clause.

• Extend SQL validation to allow queries beginning with WITH, VALUES, or TABLE
• Introduce ALLOWED_SQL_OPERATORS and update validation logic
• Add a test case to ensure DataFrame.readSqlQuery handles WITH clauses correctly

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
dataframe-jdbc/src/test/kotlin/org/jetbrains/kotlinx/dataframe/io/h2/postgresH2Test.kt	Refactored SQL string trimming and added a new WITH-clause test
dataframe-jdbc/src/main/kotlin/org/jetbrains/kotlinx/dataframe/io/readJdbc.kt	Added ALLOWED_SQL_OPERATORS and updated query-start validation logic

Comments suppressed due to low confidence (4)

dataframe-jdbc/src/test/kotlin/org/jetbrains/kotlinx/dataframe/io/h2/postgresH2Test.kt:380

• The test verifies the row count and names of high earners, but it doesn't assert the corresponding salary column values. Adding assertions for the salary values will ensure both columns are validated.

            resultDataFrame[0][0] shouldBe "Alice"

dataframe-jdbc/src/main/kotlin/org/jetbrains/kotlinx/dataframe/io/readJdbc.kt:566

• This documentation is outdated as the validation now allows queries starting with WITH, VALUES, and TABLE in addition to SELECT. Please update the doc comment to reflect the updated allowed operators list.

 * Validates if the SQL query is safe and starts with SELECT.

dataframe-jdbc/src/test/kotlin/org/jetbrains/kotlinx/dataframe/io/h2/postgresH2Test.kt:104

• The SQL string assigned to createTableStatement is already trimmed using trimIndent(). The additional .trimIndent() call on execution is redundant and can be removed to simplify the code.

            connection.createStatement().execute(createTableStatement.trimIndent())

dataframe-jdbc/src/test/kotlin/org/jetbrains/kotlinx/dataframe/io/h2/postgresH2Test.kt:125

• Similarly, createTableQuery is trimmed at assignment, making the .trimIndent() call here redundant. Consider removing it to avoid unnecessary repetition.

            connection.createStatement().execute(createTableQuery.trimIndent())