feat: no SMS & Whatsapp double signup validation

[jonathansberry]

Description

As requested by the client, ensures no users can sign up to both SMS and Whatsapp notifications.

Checklist

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• The GitHub ticket for this issue has been updated to "Ready to Review" or equivalent.
• I have developed these changes in discussion with the appropriate project manager.
• My code follows the general Fjelltopp documentation (see Confluence).
• I have made corresponding changes to the Fjelltopp documentation (see Confluence).
• I have rebased this branch with master.
• New dependency changes have been committed.
• I have added automated tests that prove my fix is effective or that my feature works.
• New and existing tests pass locally with my changes.
• My changes generate no new warnings.
• I have performed a self-review of my own code.
• I have assigned at least one reviewer.
• I have assigned at least one label to this PR: "patch", "minor", "major".