Fix deletion visibility in region migration IT

[Caideyipi]

Backport of b35a575 / #18065 to dev/1.3.

Notes:

• Applied the tree-model region migration IT and shared file assertions.
• Omitted the table-model IT from the original master change because dev/1.3 does not have the table-model test APIs/classes.

Validation:

• git show --check HEAD
• checked no Java 9+ Set.of / no-arg orElseThrow() usages remained in the backported files
• Maven spotless/test-compile could not run locally because the JVM could not reserve native memory/pagefile on this machine.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.07%. Comparing base (59243c5) to head (c4afcc5).
⚠️ Report is 2 commits behind head on dev/1.3.

Additional details and impacted files

@@              Coverage Diff              @@
##             dev/1.3   #18072      +/-   ##
=============================================
- Coverage      43.08%   43.07%   -0.01%     
  Complexity       241      241              
=============================================
  Files           3625     3625              
  Lines         242467   242467              
  Branches       29569    29569              
=============================================
- Hits          104457   104434      -23     
- Misses        138010   138033      +23     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.