Guard against element type mismatch at document type creation

[kjac]

Prerequisites

• I have added steps to test this contribution in the description below

Description

#16421 introduced guards against (and warnings for) element type mismatch for document types. Unfortunately, while the PR actually added guards at document type creation time, these guards were never actually applied 😢

This PR fixes it.

Testing this PR

You'll need to use Swagger UI to test this, as the backoffice client UI doesn't support creating inherited document types yet.

1. Create a document type.
2. Using Swagger UI, attempt to create a child element type to the document type.
3. Verify that this fails with a "mismatched" error status.

Now do the same, but with a parent element type and a child document type. The result should be the same.

You can use the following payload when testing with with Swagger UI. Remember to:

• Replace the parent document type key.
• Add your preferred document/element type alias (remember, no duplicates when testing).
• Set isElement according to the test case you're executing.

{
    "allowedTemplates": [],
    "defaultTemplate": null,
    "cleanup": {
        "preventCleanup": false,
        "keepAllVersionsNewerThanDays": null,
        "keepLatestVersionPerDayForDays": null
    },
    "allowedDocumentTypes": [],
    "compositions": [{
            "documentType": {
                "id": "[insert parent document type key here]"
            },
            "compositionType": "Inheritance"
        }
    ],
    "alias": "[replace me]",
    "name": "Inherit Test",
    "description": null,
    "icon": "icon-bug color-red",
    "allowedAsRoot": false,
    "variesByCulture": false,
    "variesBySegment": false,
    "collection": null,
    "isElement": [true/false],
    "properties": [],
    "containers": []
}