fix(scenarios): Handle non-numeric 'set' query parameter gracefully

[sentry]

The /scenarios/ page previously crashed with a ValueError (resulting in a 500 error) if the set query parameter in the URL was not a valid integer (e.g., ?set=4%5C). This occurred because the value was passed directly to ScenarioSet.objects.filter(pk=...) without prior validation.

This change introduces a check to ensure that the set parameter is numeric (isdigit()) before attempting to use it for a primary key lookup. If the parameter is not a valid number, the selected scenario set will remain None, causing the page to render the list of all scenario sets instead of crashing. This approach mirrors the existing validation logic for the version query parameter.

Fixes SIMPLEAUDITSTUDIO-2H

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