Make workshops commentable

[maebeale]

What is the goal of this PR and why is this important?

• Make workshops commentable following the same pattern as people, users, organizations, and event registrations
• Fix several bugs in the shared comment fields partial

How did you approach the change?

• Added polymorphic has_many :comments association and accepts_nested_attributes_for to Workshop model
• Added workshop lookup to CommentsController#set_commentable
• Nested comment routes under workshops
• Added admin-only comments section to workshop edit form (only shown for persisted records), matching the people form pattern
• Fixed created_by/updated_by not persisting on nested comment attributes by using assign_attributes + manual user assignment before save
• Removed leftover debug "test" text from _comment_fields.html.erb
• Changed unknown user fallback from "Unknown" text to "--" in a chip
• Fixed two-user initials chips to split 50% width (w-1/2 text-center)
• Wrapped images section in a collapsible toggle (expanded by default)
• Added workshop polymorphic association test and :for_workshop factory trait

UI Testing Checklist

• Edit a persisted workshop as admin — comments section appears
• Add a comment — created_by is set to current user
• Edit a comment — updated_by is set to current user, shows both initials chips
• Comments with unknown authors show "--" chip
• Images toggle collapses/expands correctly
• Non-admin users do not see comments section

Anything else to add?

• Dropdown toggle persistence (localStorage) was deferred to a separate PR

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