[#7109] Allow Check activities to configure one or more allowed abilities

[thatlonelybugbear]

Closes #7109
Adds support for configuring multiple allowed abilities on Check activities, and uses that configuration to constrain skill/tool check roll dialogs opened from those activities.

Previously, a Check activity could configure associated skills/tools, but only one ability. The generated chat buttons used that configured/default ability, while the roll configuration dialog still allowed selecting any ability. This could make the activity appear to request a specific ability check while still allowing the final roll to be changed freely.

This PR changes Check activity check.ability to match the existing save activity pattern by storing it as a SetField, allowing one or more abilities to be selected.

Included changes:

• Allows Check activities to select multiple abilities.
• Generates chat buttons for each selected ability and associated skill/tool combination.
• Preserves existing default behavior when no ability is selected:
  • skills use their default ability for chat button labels;
  • tools use their default/item ability for chat button labels;
  • the roll dialog still allows any ability.
• Passes configured activity abilities through skill/tool roll configuration.
• Filters the skill/tool roll dialog ability dropdown when allowed abilities are provided.
• Adds DataModel migration for legacy check.ability string data, converting it to the new array/set-compatible shape.
• Updates type documentation for the new Set<string> shape.