JDBC testing against multiple isolation levels

### Is your feature request related to a problem? Please describe.

The change #2219 has been created to fix an NPE that happens when JDBC is used with _serializable_ isolation level.

We need a thorough, JDBC oriented, set of tests that exercise against various isolation levels (read-commtted & serializable) against H2 _and_ Postgres. The tests should not only cover the case of #2219 but more generally "all" scenarios, including "interleaved" concurrent changes.

### Describe the solution you'd like

_No response_

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

Read change #2219 first, then locate the existing JDBC tests and determine how they exercise isolation levels on H2 and Postgres. Done means a thorough test set covering read-committed and serializable isolation, including the #2219 case and interleaved concurrent changes.

Written by the indexing model from the issue text.