Follow-up optimization to WT-18125.

Background

WT-18125 fixes a cache stall where pages on a stale disaggregated read-only btree (WT_BTREE_DISAGGREGATED | WT_BTREE_READONLY with an OUTDATED dhandle) could get stuck in eviction. The fix handles these pages at eviction time in __wt_evict: if the page is not clean-evictable and a reader still holds the handle, keep it resident (return EBUSY); otherwise force a clean discard so it is not routed to a dirty split that can never write back to shared storage.

Improvement

The eviction walk currently queues such pages and lets the eviction worker attempt and fail them. As an optimization, the walk could detect a stale disaggregated btree and skip queuing pages that are known not to be evictable, before they are added to the eviction queue. The natural place is a condition in __wt_page_can_evict (checked at the "fast" label in the walk's queue path), or an earlier skip alongside the existing per-tree skip logic in the eviction walk.

Note

Skipping at walk time complements, but does not replace, the _wt_evict handling: urgent eviction can still trigger eviction of a page even when it was never queued by the walk, so the in-_wt_evict path must remain.

Reviewer suggestion (PR #14276): add a condition in page_can_evict or similar before the page is added to the queue; expected to be a small change.