fix: Winforms design mode detection regressed

[xackus]

What kind of change does this PR introduce?

Fix #4368 and add a regression test.

What is the new behavior?

See description of #4358.

What is the current behavior?

See description of #4358.

What might this PR break?

See description of #4358.

Checklist

• I have read the Contribute guide
• Tests have been added or updated (for bug fixes / features)
• Docs have been added or updated (for bug fixes / features)
• Changes target the main branch
• PR title follows Conventional Commits

Additional information

In the original PR I didn't add a regression test because we can't run the Winforms designer in a test.
However, it turns out it's actually really easy to mock a site in design mode.

[glennawatson]

SonarCloud failing is expecting, since you're not part of the organisation. So it's to protect the token.

So once the CodeQL passes we can merge it in, which will be available for the next release.