Topology checks are skipped when the project uses buffered transaction groups

### What is the bug or the crash? The topology checks configured on a layer are skipped when the project uses buffered transaction groups. The checks work as expected with automatic transaction groups. Overlapping polygons, for example, can be saved without anything happening: no message bar, no entry in the Geometry validation panel, no block. This is contrary to the [QGIS Documentation](https://docs.qgis.org/4.2/en/docs/user_manual/working_with_vector/vector_properties.html#topology-checks), which states: "Topology checks will be executed when the user saves the layer. Check errors will be reported in the Geometry validation panel. As long as validation errors are present, the layer can not be saved". ### Steps to reproduce the issue 1. Open a project with an editable polygon layer (tested: PostGIS and GeoPackage) 2. Project > Properties > Data Sources > Transaction mode: "Buffered transaction groups" 3. Layer Properties > Digitizing > Topology checks: enable "Overlap" 4. Toggle editing, drag a vertex of one polygon into a neighbouring polygon so that the two overlap 5. Save layer edits: the edit is saved without any complaint, and the Geometry validation panel stays empty. ### Versions I reproduced the issue with 3.40.9 and 4.2.1.

QGIS-Version	4.2.1-Belém do Pará
QGIS-Codeversion	f229b86a9c1
Bibliotheken
Qt-Version	6.11.1
Python-Version	3.12.13
GDAL-Version	3.13.2 — Iowa City
PROJ-Version	9.8.1
EPSG-Registraturdatenbankversion	()
GEOS-Version	3.14.1-CAPI-1.20.5
SFCGAL-Version	2.3.0
GeographicLib-Version	2.7.0
SQLite-Version	3.53.4
PDAL-Version	2.10.0
PostgreSQL-Client-Version	18.4
SpatiaLite-Version	5.1.0
QWT-Version	6.3.0
QScintilla2-Version	2.14.1
OpenGL-Version	4.3 (Kompatibilitätsprofil), Intel(R) Iris(R) Xe Graphics
BS-Version	Windows 11 Version 24H2
Aktive Python-Erweiterungen
QgisModelBaker	v8.3.10
db_manager	0.1.20
linking_relation_editor	v1.5.0
processing	2.12.99
solocator	2.0.0

### Supported QGIS version - [x] I'm running a supported QGIS version according to [the roadmap](https://www.qgis.org/en/site/getinvolved/development/roadmap.html#release-schedule). ### New profile - [x] I tried with a new [QGIS profile](https://docs.qgis.org/latest/en/docs/user_manual/introduction/qgis_configuration.html#working-with-user-profiles) ### No Duplicate of the Issue - [x] I have verified that there are no existing issues raised related to my problem. ### Additional context _No response_