Add test_case1_sequential_workspace_migration to verify the fix for
the multi-tenant data loss bug in PostgreSQL Case 1 migration.
Problem:
- When workspace_a migrates first (Case 4: only legacy table exists)
- Then workspace_b initializes later (Case 1: both tables exist)
- Bug: Case 1 only checked if legacy table was globally empty
- Result: workspace_b's data was not migrated, causing data loss
Test Scenario:
1. Legacy table contains data from both workspace_a (3 records) and
workspace_b (3 records)
2. workspace_a initializes first → triggers Case 4 migration
3. workspace_b initializes second → triggers Case 1 migration
4. Verify workspace_b's data is correctly migrated to new table
5. Verify workspace_b's data is deleted from legacy table
6. Verify legacy table is dropped when empty
This test uses mock tracking of inserted records to verify migration
behavior without requiring a real PostgreSQL database.
Related: GitHub PR #2391 comment #2553973066
0fb7c5bc3b