fix(db): commit session in CheckpointRepository.create to prevent GC-induced data loss

Without an explicit session.commit(), the session created by create()
could be garbage-collected before prune() runs its query, causing the
StaticPool connection to be returned to the pool and the pending
transaction to be rolled back. This made prune() see an empty table
and return [] instead of the expected pruned IDs.

Adding session.commit() ensures the checkpoint row is durably written
before the session goes out of scope, fixing the flaky
CheckpointRepository prune test (features/db_repositories_cov_r3.feature:292).

ISSUES CLOSED: #8759

src/cleveragents/infrastructure/database/repositories.py

@@ -5628,6 +5628,7 @@ class CheckpointRepository:
             model = CheckpointModel.from_domain(checkpoint)
             session.add(model)
             session.flush()
+            session.commit()
             return model.to_domain()
         except IntegrityError as exc:
             session.rollback()