Builds: add tests for delete_old_build_objects and fix KeyError in _delete_builds

[Copilot]

• Write tests for delete_old_build_objects function
• Fix KeyError in _delete_builds when no builds are deleted
• Use assert foo == bar syntax instead of self.assertEqual in TestDeleteOldBuildObjects

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[stsewd]

@copilot code review[agent] use the assert foo == bar syntax

[Copilot]

@copilot code review[agent] use the assert foo == bar syntax

Done in 724f4c2 — replaced all self.assertEqual calls in TestDeleteOldBuildObjects with assert foo == bar syntax.