[Fixes #14377] Honor skip_existing_layers flag

[ostyhar]

Fixes #14377.

The upload API exposes skip_existing_layers, while the importer checks skip_existing_layer. This change normalizes the flag before the execution request is created and adjusts the expected layer count when existing layers are skipped.

It also fixes the raster and 3D Tiles skip paths and adds regression tests for full and partial skips.

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[ostyhar]

The CLA was sent to info@osgeo.org

[mattiagiupponi]

Hi @ostyhar
I dont agree much with the implementation, i would suggest to approach it in a different way:

1. the main issue since the parameters is skip_existing_layer instead of the one in the serializer skip_existing_layers i agree that we should do it, but I think the change should be done on the extract_params_from_data for each handler rather than an IF in the view. Understandable that at least is common for everyone, but goes againts the principle to have each handler responsible for the preparation of the parameters required for the execution
2. Rather than leaving the funciton as it is and move logic in the datastore to evaluate the execution, since we want to skip some dataset, i suggest to move the control should_be_imported into the _select_valid_layers which checks if the layers are valid. Ofc if i need to skip it is not valid, then should be removed from the selected valid layers. We can add a log into the function to evaluate it. In this way when it reaches the import_resource there are not valid layers, so then the execution stops.