SlickProjectionSpec: A Slick grouped projection must restart from previous offset - handler throwing an exception, save after 2

https://github.com/apache/pekko-projection/actions/runs/10040359450/job/27746257858

Notably:

```
org.h2.jdbc.JdbcSQLIntegrityConstraintViolationException: NULL not allowed for column "CONCATENATED"; SQL statement:
merge into "TEST_MODEL" ("ID","CONCATENATED") values (?,?) [23502-224]
at org.h2.message.DbException.getJdbcSQLException(DbException.java:520)
```
and
```
[info] check: last seen offset is 2L The Option on which value was invoked was not defined. (SlickProjectionSpec.scala:953)
[info] org.scalatest.exceptions.TestFailedException:
[info] at org.scalatest.OptionValues$OptionValuable.value(OptionValues.scala:114)
[info] at org.apache.pekko.projection.slick.SlickProjectionSpec.f$proxy17$1$$anonfun$4(SlickProjectionSpec.scala:953)
[info] at org.scalatest.Assertions.withClue(Assertions.scala:1208)
[info] at org.scalatest.Assertions.withClue$(Assertions.scala:421)
[info] at org.apache.pekko.actor.testkit.typed.scaladsl.ScalaTestWithActorTestKit.withClue(ScalaTestWithActorTestKit.scala:41)
[info] at org.apache.pekko.projection.slick.SlickProjectionSpec.f$proxy17$1(SlickProjectionSpec.scala:955)
[info] at org.apache.pekko.projection.slick.SlickProjectionSpec.$init$$$anonfun$3$$anonfun$5(SlickProjectionSpec.scala:923)

```

Start with SlickProjectionSpec.scala around lines 923-955 and the linked CI run, focusing on the grouped projection restart after offset 2. Reproduce the H2 NULL constraint failure and the missing Option at line 953. Done means the projection restarts from the previous offset without the handler exception and the test passes.

Written by the indexing model from the issue text.