Fix UI workflow delete 404 by wiring cleanup route

[santoshkumarradha]

Summary

• register missing UI cleanup route: DELETE /api/ui/v1/workflows/:workflowId/cleanup
• add route regression test to ensure the cleanup endpoint is wired and reachable
• make stub cleanup response non-nil in route tests so handler assertions are meaningful

Root cause

The frontend correctly called /api/ui/v1/workflows/:id/cleanup?confirm=true, but setupRoutes() never registered that route, so Gin returned a framework-level 404 before CleanupWorkflowHandler executed.

Validation

• go test ./internal/server -run "TestSetupRoutesRegistersWorkflowCleanupUIRoute" (from control-plane/)

Issue

Closes #173