Background

WT-18626 fixed a stale-image bug in the disaggregated victim cache. The fix introduced *evict_page_disagg_image() to resolve the correct on-disk image for a page's current *disagg_info->block_meta. That helper is covered by the new Catch2 test in PR #14717.

Goal

Add eligibility-level unit tests that exercise __evict_page_victim_cache_eligible() directly (via a new HAVE_UNITTEST wrapper if needed) and verify that it returns false and leaves the out image NULL for all cases that must not enter the victim cache:
* non-disaggregated btree
* checkpoint-cursor btree
* internal page
* root page
* cold-storage-tier btree
* disagg_info == NULL
* invalid page_id
* reconciliation result is split / empty / replace without retained image

Acceptance Criteria

• New Catch2 test file under test/catch2/misc_tests/ registered in test/catch2/CMakeLists.txt.
• Tests fail on a build where the eligibility check reverts to the pre-WT-18626 logic, and pass with the fix.
• Ticket linked to WT-18626 as "Relates to".