Parse the Contagion Test dialog form into a ContagionTestChoice. Pure and Foundry-free so it can be unit-tested.
The parsed dialog form data.
The candidate afflictions; the <select> option values are their shortcodes.
<select>
The parsed choice, or null when the selected shortcode is unknown.
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Parse the Contagion Test dialog form into a ContagionTestChoice. Pure and Foundry-free so it can be unit-tested.