Errors inside a transaction lose SQLState and the vendor code #109

In auto-commit mode, executeOneShot throws LibsqlException, and LibsqlErrorMapper converts it. Inside a transaction, a StreamResult.Error becomes new SQLException(err.error().message()). The SQLState, the vendor code, and the constraint categorization are lost.

The same failing INSERT, for example a unique violation, produces a different exception type depending on autoCommit. Since #64, every connection with setAutoCommit(false) takes the unmapped path. Frameworks that branch on SQLState behave differently.

Sites:

• LibsqlConnection.executeStatement
• beginTransaction
• endTransaction
• extractBatchResults

Expected:

• Route HranaError through the mapper at all four sites.
• Add a test that checks the exception type and SQLState for a unique violation in both modes.