mapit_filter: Unable to filter by type and code

Problem

When importing using mapit_import you can’t import codes while also filtering for an area type:

If you specify both --code_field/--code_type and --area_type_code then the area type is ignored when searching for existing areas.
If you specify only --area_type_code then the codes are not imported.

When one code is used in multiple area types this can lead to areas of a different type being modified.

Steps to reproduce

I observed this when importing the WMCF areas because there are a few scottish constituencies which keep their area code. Importing them with:

python manage.py mapit_import --area_type_code=WMCF --country_code=first-letter --name_type_code=O --code_field=gss_code --code_type=gss parl_constituencies_2025.gpkg

Will only create 632 WMCF areas although there are 650 areas in the dataset. Turns out the culprit is this bit of code in mapit/add_areas_from_file/core.py:

            if parameters.new:  # Always want a new area
                raise Area.DoesNotExist
            if code:
                matching_message = "code %s of code type %s" % (
                    code,
                    parameters.code_type,
                )
                areas = Area.objects.filter(
                    codes__code=code, codes__type=parameters.code_type
                ).order_by("-generation_high")
            else:
                matching_message = "name %s of area type %s" % (
                    name,
                    parameters.area_type,
                )
                areas = Area.objects.filter(
                    name=name, type=parameters.area_type
                ).order_by("-generation_high")

Workaround

On the initial import use --preserve to force creating new areas.
No workaround for updating areas.

Proposed change

If both codes and an area type is specified when importing the script should filter by both. In particular it should never touch areas of another type than the one specified as --area_type_code.

mysociety / mapit