[BUG] viewStates prune boundary (missing updatedAt, ties) and dispose retention (#1065) have no regression coverage

[easonLiangWorldedtech]

Bug description

The durable per-view viewStates pipeline (vps2 series, #1546) is pinned by a single well-formed-entries prune baseline test (ClineProvider.spec.ts:1210). Three boundary behaviors are untested:

1. An entry without updatedAt (written before the field existed) ranks last via updatedAt ?? 0 (ClineProvider.ts:634) and is the first to fall off the 50-entry cap — untested.
2. Ties on updatedAt preserve insertion order (stable sort), so the first 50 registered views survive the cap — untested.
3. dispose() is retention-only for durable per-view state: a view's viewStates entry must survive provider teardown so an editor tab reopens with its saved selections (Preserve durable editor view state across provider disposal #1065) — untested, so a future teardown rewrite that clears the entry would pass CI.

Expected behavior

Pin all three with regression tests:

• Pruning a 51-entry map (50 timestamped + 1 without updatedAt) keeps 50 entries and drops the updatedAt-less entry.
• Pruning 55 entries sharing one updatedAt keeps the first 50 inserted and drops the last 5.
• After setViewStateId + saveViewState, dispose() leaves the durable entry in viewStates intact.

Reproduction

No failing behavior to reproduce — this is a coverage gap: a dispose() implementation that clears the view's entry (or a prune that drops ties arbitrarily) would pass the existing suite.

Part of the vps2 durable per-view state series — tracked in easonLiangWorldedtech#41.