Added readOnly mode

[zaleslaw]

Fixes #1308

[Copilot]

Pull Request Overview

Introduces a new read-only mode for managed JDBC connections and refactors DataFrame read methods to use it.

• Add readOnly flag to DbConnectionConfig and implement withReadOnlyConnection helper for safe rollback semantics.
• Refactor DataFrame.Companion JDBC read methods (readSqlTable, readSqlQuery, etc.) to use the new helper.
• Add an H2 integration test for withReadOnlyConnection.

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/h2Test.kt	Add test for withReadOnlyConnection behavior, including auto-commit disabling and rollback.
dataframe-jdbc/src/main/kotlin/org/jetbrains/kotlinx/dataframe/io/readJdbc.kt	Introduce DbConnectionConfig.readOnly, withReadOnlyConnection, and update DataFrame read methods to use managed connections.

Comments suppressed due to low confidence (4)

dataframe-jdbc/src/test/kotlin/org/jetbrains/kotlinx/dataframe/io/h2/h2Test.kt:1160

• Test name indicates it sets read-only mode; consider adding an assertion like conn.isReadOnly shouldBe true to verify the read-only flag is applied.

            conn.autoCommit shouldBe false

dataframe-jdbc/src/main/kotlin/org/jetbrains/kotlinx/dataframe/io/readJdbc.kt:129

• The docs claim that data-modifying queries are rejected when readOnly=false, but no enforcement logic is implemented. Consider either implementing a statement filter or updating the documentation to reflect the actual behavior.

 *                    but the library will still reject any queries that appear to modify data

dataframe-jdbc/src/test/kotlin/org/jetbrains/kotlinx/dataframe/io/h2/h2Test.kt:1155

• Missing import for DbConnectionConfig in this test file, causing a compilation error. Add import org.jetbrains.kotlinx.dataframe.io.DbConnectionConfig.

        val config = DbConnectionConfig("jdbc:h2:mem:test;DB_CLOSE_DELAY=-1", readOnly = true)

dataframe-jdbc/src/main/kotlin/org/jetbrains/kotlinx/dataframe/io/readJdbc.kt:221

• [nitpick] The default read-only behavior documentation is duplicated across several methods. Consider extracting this common description into a shared section or referencing a central doc to reduce duplication.

 * ### Default Behavior: