DashboardQuery specs: add a query-count guard and host-less workshop coverage #3071

The spec added in PR #3066 checks host values only, so it cannot detect a query-shape regression on the dashboard path. Two gaps to close in spec/queries/dashboard_query_spec.rb:

Query-count guard. The new example asserts map(&:host)).to all(eq(host)). Workshop#host lazy-loads through workshop_host&.sponsor, so deleting preload(workshop_host: :sponsor) from app/queries/dashboard_query.rb leaves every dashboard spec green while restoring a per-card query on the homepage. The events path already guards this at spec/requests/event_card_render_query_cost_spec.rb:38 ("does not add per-card queries for hosted workshops on /events/upcoming"); the dashboard needs a counterpart that asserts the query count stays flat as the card count grows.

Host-less workshops. Nothing asserts the other half of the #2975 failure mode:

• a workshop with only non-host sponsors must resolve host to nil, not to one of those sponsors
• a workshop with no workshop_sponsor rows must render without breakage

Raised from the review of PR #3066. Related: #2982, #2975.