display_groups/sync: cover Deref + IntoIterator impls (#562)

* display_groups/sync: cover Deref + IntoIterator impls

Adds three tests exercising the previously-uncovered trait impls
on DisplayGroupSubscription:
- Subscription<T>::next reached via Deref::deref
- (&sub).into_iter (the borrowed IntoIterator impl)
- sub.into_iter (the owned, consuming IntoIterator impl)

Extracts a stubbed_subscription helper + display_group_update_response
encoder so the four tests share fixture construction.

Coverage 76% → 96% lines, 82% → 96% regions, 50% → 100% functions.
The 3 remaining lines are panic! arms inside let-else patterns,
unreachable when tests pass.

* /simplify pass: named constants, MessageBusStub::with_responses, dedupe

- stubbed_subscription: replace inline MessageBusStub literal with
  MessageBusStub::with_responses (existing constructor)
- display_group_update_response: derive from IncomingMessages::DisplayGroupUpdated
  and TEST_REQ_ID_FIRST instead of magic 68 / 9000
- Client::stubbed: server_versions::LINKING (the actual gate per C#
  EClient.cs:3449) instead of magic 176
- Drop the now-redundant WHAT comment on display_group_update_response
- Collapse 3 near-identical test bodies via assert_first_data_eq helper
- Hoist top-of-mod use for SubscriptionItem (was triplicated per test)

Side-effect: coverage 96% → 98.6% lines because helper consolidation
reduced instrumented sites.

Async sibling display_groups/async.rs has the same 176/9000/68 warts
but is out of scope; can be aligned in a separate sweep.