Invalid operateURL not handled

[Buckwich]

Describe the bug

modeler hangs if an invalid operate URL is configured for task testing

Steps to reproduce

1. configure a valid connection with an invalid operateURL (eg localhost:8088/operate)
2. try to run task testing (the only place where that url is used, currently)

Observe: testing never completes, loading state, console error

Expected behavior

no crash

Environment

• Version: 5.44.0
• Operating System: Mac OS 10.15.7
• Plugins:
• Execution Platform: BPMN - Camunda 8

Additional context

• identified by Copilot in fix: use configured operateUrl for startInstance #5669 (comment)