fix: option to test schema migration with random subset of corpus

[nayib-jose-gloria]

Reason for Change

• Test-only bug; previous implementation to run random subset of corpus did not account for filtering out published collections with active revisions. This change moves the subsetting to only sample from the set of collections AFTER filtering out ineligible collections (published w/ active revisions)

[github-actions]

Deployment Summary

• backend_url: https://pr-6221-backend.rdev.single-cell.czi.technology
• frontend_url: https://pr-6221-frontend.rdev.single-cell.czi.technology
• delete_db_task_definition_arn: arn:aws:ecs:us-west-2:***:task-definition/dp-rdev-pr-6221-deletion:1
• migrate_db_task_definition_arn: arn:aws:ecs:us-west-2:***:task-definition/dp-rdev-pr-6221-migration:1

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (1e902ab) 91.60% compared to head (1872913) 91.62%.

❗ Current head 1872913 differs from pull request most recent head d27ed5a. Consider uploading reports for the commit d27ed5a to get more accurate results

Files	Patch %	Lines
backend/layers/processing/schema_migration.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6221      +/-   ##
==========================================
+ Coverage   91.60%   91.62%   +0.02%     
==========================================
  Files         175      175              
  Lines       14232    14228       -4     
==========================================
  Hits        13037    13037              
+ Misses       1195     1191       -4     

Flag	Coverage Δ
unittests	91.62% <80.00%> (+0.02%)	⬆️

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